"Find the enemy and shoot him down. Anything else is nonsense"
While it would stand to reason that most games of warfare in the far future would be completely based around weapons that fire at each other, this is not the way to have a fun game. Shooting is only one possible strategy.
Before you can decide to simply sit and shoot at your opponent, you need to know if doing so is a wise decision. Afterall, if your opponent can sit back and shoot at you, with a better expected result, you're going to lose the game. So lets examine what factors go into determining which army (or unit) is more likely to win a firefight engagement.
1) Range - If your opponent's guns outrange your guns, then they can find a place to sit and shoot you, while you cannot shoot back. For formulating a strategy, you want to consider the overall army range, not just one or two units. If all of your units can shoot 36 inches, while one of your opponent's units can shoot 48 inches, and the rest are limited to 24, you have the overall range advantage. Yes, you will probably take a shot or two as you close from 48 to 36 inches, but after that, you can bring your full strength to bear, and he cannot. Also important here is effective range. It is of no importance if your opponent can shoot 72 inches if you are playing on a 48 inch board. Everything over 48 inches is inconsequential. Likewise, a board with lots of terrain that blocks line-of-sight can also reduce the effective range. If the longest straight line that is unblocked by terrain is 27 inches, then any weapon ranges over 27 inches should be thought of as 27 for purposes of knowing which army has the range advantage.
2) Numbers of shots that are likely to hit - If you have more shots than your opponent, you have an advantage over them. This isn't as easy as counting models, however, as some models can make 6 shots a round. So, you need to know how many shots you get, not how many models. Included here is the ability of your troops to hit with the weapons they have. Shooting 60 shots but hitting with 10 is worse than shooting 20 shots and hitting with 13.
3) Quality of shots - If your shots are generally higher strength and lower AP than your opponent's shots, you have the quality of shot advantage.
4) Ability to take a hit - While this doesn't appear to play into the shooting calculation, it does, because it determines how likely they are to be able to fire back after you shoot them. Troops that cannot fire back are a non-factor, while something like a wraithlord or a monolith, which can absorb significant fire before being removed, is going to continue firing at you turn after turn.
If your opponent has significantly more shooting power than you, your tactical options are reduced. You have to either defeat them in close combat, or you have to be able to outmaneuver them to be able to destroy parts of their force without risking being fired upon by their full strength. Even if you designed a shooty army, if your opponent is more shooty, standing and firing will make you lose.
If you have more firepower than your opponent, you have to expect them to attempt to the same, and so your goal is to prevent them from isolating your forces or assaulting you.
If you are roughly even, then you can trade shots and hope for the best, or try to set up various situations where you can reduce their firepower without sacrificing yours. Standing and shooting is still an option, but you will want to get tactical advantages to increase the likelyhood that your army will come out on top.
For an example of deciding who is shootier, we can examine four very basic armies, each at the 100 point level.
What can we deduce from this information?
Range: the Tau basic trooper has the edge, but each other army has a special weapon with a slightly longer range than them. The eldar, although getting 3 starcannon shots, don't get to use the rest of their guns outside of the 12 inch range. (Tau, Orks, Marines, Eldar)
Number of shots expected to hit: The Marines can be expected to land 4 shots a turn. The Firewarriors, 5. The eldar, when in range, 7 or 8, but all the other armies get double shots at that range too, so 1. The orks, around 5. (Tau, Orks, Marines, Eldar)
Strength of shots: Overall, the Tau have the highest strength weapon, although the eldar and marines' special weapons are stronger. The Ork's special weapon is only as good as the tau basic gun. (Tau, Marine, Eldar, Ork)
Ability to take a hit: Although the marines have the least wounds, they have the highest toughness and best armour. The orks have higher toughness, but don't get a save at all against any of the other guns. The eldar also don't get a save against any of the other guns, plus they have a lower toughness. (Marine, Tau, Ork, Eldar)
Given that the marines and tau are consistently near the top here, we can run the numbers to examine even closer which army has the firepower advantage. I will be giving some expected result numbers, without showing all my work. The work is left as an exercise for the reader (who, if they're not yet comfortable with this, should actually work it out to get practice.)
Over 30 inches, the marines have a rocket shot, but the Tau have nothing. One rocket is likely to kill .55 Tau.
Between 30 and 24 inches, the marines still have one rocket, the Tau have 10 shots, likely to kill 1.11 marines.
Between 12 and 24 inches, the marines have a rocket, and 5 bolters, for a total of 1.66 kills. The Tau would still kill 1.11 marines at this range.
Between 0 and 12 inches, the marines have a rocket and 10 rapid-fire bolters, giving 2.77 kills, while the Tau would get 20 rapid-fire pulse rifles, killing 2.22 marines.
As you can see, the Tau have a six inch "sweet spot" between 24 and 30 inches, where they can outshoot a similar number of marines. But, the marines have a slight advantange both closer than 24 inches, and further than 30 inches. Overall, the numbers are very similar. Whoever fires first is going to end up with the advantage as the number of shots being returned will decrease.
Comparing the marines to the orks however, yields a different picture.
Over 36 inches, the marines rocket is still likely to result in .55 kills. From 36 inches to 24 inches, the marines still get their rocket, the orks get 6 big shoota shots, which can be expected to kill .44 marines. Between 12 and 24 inches, the marines bolters add in, for a total of 2.22 kills. The orks manage a total of .88 marines In rapid-fire range, the marines kill 3.88 orks, while the orks get 1.32 kills.
The marines manage a 2-to-1 ratio in all but the 24-36 range. The marine's superior skill and armour wins over the orks numbers easily, so the orks are forced to find another way to win the battle. (Besides, standing around shooting isn't very orky).
The basic idea behind shooting is to inflict more casualties than you take. The use of tactics to gain local superiority is the method used to accomplish this.
See, when we consider our overall strategy, we look at the big picture, and decide who has overall superiority in terms of mobility, firepower, and assault. But, even if your opponent has overall superiority, you can still set up situations where you have local superiority. That is to say, that if you look at a smaller portion of the board, in some area, you have more firepower than your opponent.
Local superiority is achieved most easily by taking your best shooting troops, and massing them together in one location. In that way, your opponent cannot easily engage you in a firefight at that location, as you will have significant firepower. You do, however, need to use have significant firepower as to be able to create such a threat that your opponent will respect. This is generally called a firebase, and it is used to give covering fire to the other elements of your army. More often than not, a firebase is centered around your heavy support units, but this isn't always the case.
A good firebase will have the following characteristics:
For example, the eldar can field a squadron of 3 War-Walkers as a Heavy Support option. Each War-Walker can be given 2 starcannons, so the unit can be equipped with a total of 6 starcannons, or 18 strength 6, AP2 shots a turn. That sort of firepower will even cut down squads of terminators with ease, making it an excellent firebase.
If you have a significant enough firebase, with good range, you can force your opponent to keep their troops out of line-of-sight of the firebase, and by doing so, allow other units in your army to advance along those lines relatively unmolested.
It should be obvious, but it bears saying, that if you have weapons that can hit your enemy, while you are out of range of their guns, your have local superiority, and should exploit it until they move either towards you or away from you. Of course, if they move towards you, they may have superiority after only a move or two.
Example: A group of 8 marines are fighting 12 Tau Firewarriors. (Both 120 points) They start 30 inches apart, and the Tau go first. They have the range to shoot the marines, so they do, killing one marines. The Marines can either run away, or advance 6". They advance 6 inches, and the next turn, the Tau kill another marine. The Tau's range gave them 2 turns of firing on the marines before the marines are able to return fire, now with diminished numbers.
If you have the ability to bring more troops to bear in a subsection of the board, you will have local shooting superiority. I mentioned before that current U.S. military doctrine states that in order to engage the enemy, you should have a three-to-one numerical advantage. Let's see how that plays out in the 40k world.
The enemy shall be the omnipresent squad of 10 marines with bolters. Our forces will be an equal number of points worth of eldar guardians (19).
With one-to-one odds, the eldar can be expected to kill slightly over 3 marines. (19 eldar * 2 shots each * 1/2 hit * 1/2 wound * 1/3 failed saves) That's ok, but it leaves 7 marines to shoot back the next turn, and those 7 marines would kill 6 guardians on their turn. 3 marines cost 45 points, while 6 guardians cost 48. At best, the eldar are fighting an even battle.
With three-to-one odds, there are now 3 squads of eldar firing. Each squad kills 3 marines, leaving one marine alive. That one marine will be forced to take leadership tests, all-on-his-own tests, and, even if he's brave (stupid) enough to stick around, he's only likely to kill one eldar in return. This is a much more decisive victory for the eldar.
This sort of numerical advantage is hard to find on the gaming table, as armies are designed with equal points in mind. But, if you can get away with it, even a two-to-one advantage will be significant.
You can use mobility to bring a numerical advantage to bear on an isolated part of your opponent's army. You can maintain discipline and keep your troops together, so that they cannot be outnumbered. You can use terrain in such a fashion as to split the opponent's force. But, most of these methods rely on your opponent making the mistake of getting outnumbered, which you really cannot count on.
Tricks and Traps
This is where we include the move and fire tricks. Weapons that can move and fire (assault weapons, weapons mounted on vehicles, bikes, battlesuits, and rapid-fire and pistols under 12 inches) have an inherent advantage over those that cannot. They allow you to fire first. A unit that can remain hidden behind terrain up until the turn it fires is able to engage at full strength, whereas a unit that has to move into position and then wait a turn has to spend a turn being shot at. If enough models shoot during that first turn of firing, they can render the enemy's counter fire significantly less effective.
Some units (eldar jetbikes and warp spiders, Tau battlesuits) have the ability to move after they fire. By taking a position near terrain, they are able to move into position to fire during their movement phase, and retreat back behind cover during their assault phase. As long as none of their opponent's units are able to move behind the terrain and fire on the next turn, they have local superiority around that terrain piece.
Other units can cause pinning, or other morale checks. A unit that is pinned is unable to move or fire in their next turn, and you can shoot them again the next turn. Although pinning tests are not reliable enough to depend heavily upon, as most units will pass these tests more than half the time, they can cause situations that are difficult for your opponent to get out of.
Blocking Lines of Sight with close combat
I have left this until last, as it is somewhat complicated. Depending how your opponent has their models arranged, it can be advantageous to declare charges that you even know you may not win, as long as you are confident that you won't die right away.
If your opponent is lined up with one unit behind another, if you assault the front unit, on their next turn, they cannot shoot into, or through, the close combat. If you have a throw-away (low-point, or previously wounded) unit, you can use it to create such a temporary line-of-sight blocker, prevening the opponent's back lines from shooting at anything. You hope that you either win or lose combat at the end of your opponent's turn, so that the line-of-sight block is gone during your next shooting turn.
The key to all of these methods is the idea that you want to be able to inflict damage without taking much damage in return. This is hard, because in general, if your units can see the enemy units, the enemy units can see your units. Trading one-for-one will keep you in the game, but it won't win you any games.
So far we have talked about trying to destroy a single unit, and what that might entail. But, there is rarely only one enemy unit. Sometimes, you have to make hard choices about the order in which to deal with your opponent's units.
The best rule to apply here is the simplest one: shoot at whatever will hurt you most. And that is my grain of wisdom for the day.
What? You want more? You want to know what will hurt you the most too?
Almost without exception, fast enemy assault units are the most dangerous things that your shooter guys can face. Not only for the damage that they can do themselves (which can be considerable), but also for their ability to block lines of sight for your other units. Imagine that you have three troop units, positioned in lines, one behind the other. Ordinarily, this allows an enormous amount of fire to be delivered from their position. But, should an opponent assault the front unit, the other two units can no longer fire, as the swirling melee blocks their view of the rest of the field. Furthermore, these units are dangerous because you may only have one turn to shoot them before they assault you, so you need to reduce their numbers in a hurry. The worst thing a shooty army can face is an enemy comprised of all fast-assaulty elements.
Included in this category are enemy bikers, jump infantry, and cavalry, and open-topped transports. Fortunately, these types of units tend to be smaller or more fragile, as bikers and jump infantry can cost up to twice the points that standard infantry cost, while open-topped transports rarely have much armour and take damage easily.
Open-topped transports allow the enemy to assault after disembarking after moving. Closed transports only allow assaulting after disembarking if the vehicle has not yet moved.
If there are no fast assault units to worry about, then any full transport should be the next highest priority, as killing the transport can significantly damage the unit inside as well as reducing the opponent's mobility.
After that, any heavy firepower units, or if later in the game, any unit that may be able to assault on their turn are the standard fallbacks, followed by anything you can get a shot at, and anything on an objective you want. (Yeah, at some point, it becomes a matter of simply shoot whatever you see...)
And, always pay attention to each unit's potential other shots before deciding who will fire where. A unit with only one possible target should always fire before one with two potential targets, as the result of the first unit firing may impact your decision where to fire with the second.
Some other ideas for avoiding being assaulted
Use terrain wisely. This cannot be stressed enough. If your opponents have to move through a patch of difficult terrain to assault you, this can buy you an extra turn of shooting them. Or can cost them a couple of casualties as they crash their bikes and landings in the difficult terrain. In the best case, it can even prevent them from reaching your troops during an assault move.
Sometimes, different facets of the opponent's army will require different uses of terrain. For example, to defend against incoming enemy fire, you want terrain that will block their line of sight. But, terrain that blocks your enemies line-of-sight also blocks your line-of-sight, allowing their assault troops to close in without being shot. If you have more firepower and are generally the shooty army, it may be worthwhile setting up in the open, or at best in cover, and accepting that some enemy units will shoot you, in order to have open shots at their assault units as they cross the field.
Use vehicles to create terrain where you want it. Vehicles block both enemy unit's movement. You can take a couple of empty transports and create a funnel, allowing you to dictatate where the enemy has to move in order to close with you. Or you can attempt to tank-shock them (although, this is less likely to work, and may even block your line-of-sight to the vehicle. Be careful doing this however, as some enemy troops will have melee weapons that can destroy a vehicle, and giving them an extra six inch charge to reach the vehicle also gets them closer to your shooters.
What to do if they're going to assault anyway
Well, it's going to happen. At some point, your shooters will be assaulted. Might not happen every game, but it will happen.
First, realize that if you have a unit built for shooting prowess, being assaulted by a unit designed for assault, your guys are probably already dead. Anyone they can take down with them is a bonus. If the unit survives, it's a bonus. So, most of what you will be doing is trying to minimize the damage that they'll inflict elsewhere in your army. What follows are some possible methods for dealing with these situations.
Tie them up. If the enemy assault unit has taken enough casualties during the approach, or if they're a single model, it is possible that you can tie them up in combat, preventing them from accomplishing anything else for the rest of the game. For this to be successful, you need to know approximately how many of your guys your enemy will kill every round of combat. This will let you plan accordingly. Highly resilient units like terminators and farseers are best used for this sort of maneuver.
Example: 4 Chaos Terminators are assaulting the Eldar Farseer + his squad of 5 warlocks. Terminators are typically very formidable assault troops, armed with power weapons and power fists. However, the eldar unit has a re-rollable 4+ invulnerable save, which means that 75% of all wounds inflicted on them can be ignored. Ignoring the possibility that the eldar kill any terminators, the terminators will make 8 attacks each turn, hitting with half (4), and inflicting wounds 3/4ths of the time (a mix of power weapons and power fists). (3) But, the eldar save 75% of those wounds, meaning that on average, only .75 eldar dies. At that rate, it will take 8 combat rounds, or 4 full turns to kill the farseers unit. During that time, the eldar will probably kill a couple of terminators, which will increase their survival time even further. Even if they fail to kill all the terminators, they have prevented them from shooting at, or assaulting any other, more fragile troops. The terminators have been neutralized for the rest of the game.
Risks and drawbacks: By prolonging the combat, you do run the risk that the enemy will get pour more troops into the assault, ending it sooner than you planned. You also accept that the combat you are tying up will be around, blocking line-of-sight for a number of turns. Finally, you lose the unit that is doing the tying up. Also, if your troops have low leadership, even one casualty a turn can force them to take a morale test.
Counter-assault: This maneuver can work with any unit, but is best with a small, but dangerous assault unit that isn't fast enough to assault the enemy alone. Your opponent assaults your forward line (the shooters), but doesn't kill them all. On your turn, your assault units counter-assault into the enemy, gaining bonus attacks for charging, and having full strength against a potentially weakened enemy.
Against any army with fast assault elements, this is the best way to utilize your slower assault troops. Your guys won't ever get the charge against faster enemies, so you have to accept that your opponent will charge something, and then respond.
Risks and drawbacks: You're already accepting that your assault component isn't as mobile as your enemies. This naturally gives them the option of where to strike. An astute opponent will see the counter-assault, and direct their charge somewhere else. So, in order for this to work, you need to keep your guys somewhat close together, which can prevent other problems.
Catcher's mitt: With this tactic, you take a small inexpensive unit and advance them seven inches ahead of your firing line. Seven inches is important. When an attacking unit wipes out their enemy, they are allowed up to six inches of consolidation movement. If they contact another unit, they are considered in combat with them until the next turn. If you leave seven inches between your forward unit and your firing unit, they cannot consolidate into your firers, and this gives your shooters a good chance to get a round of rapid-firing against the victorious unit.
Risks and drawbacks: Well, you're putting a unit out as charge-bait, so expect to lose them. Don't use bait more expensive than the target you're hoping to catch! Trading 25 points of guys to kill 160 points of assaulters is good. Trading 100 points of guys to kill 100 is only break-even, and of course it gets worse from there.
The worst thing that can happen in this tactic is that your mitt holds together, leaving your enemy in combat during your shooting phase, only to have the enemy kill off the mitt at the end of your turn, leaving them free to move again on their turn. Also, against units with an exended charge range (cavalry, notably), if enemy shooting can destroy the mitt, the cavalry can charge through the space they used to occupy, and assault your shooters. So, you need to ensure that the mitt will die when you want it to.
Run away (but not too far): If you have two units of shooter guys near each other and one gets assaulted, it's time for the second group to start running. If you can get seven inches away, then if the battle ends on your opponent's turn, on your turn, you're set to fire. If the assault ends on your turn, you're pretty much screwed though. Against a unit of jump troops, the minimum safe distance from a melee is 6" (consolidate) + 12" (their movement phase) + 6" (their assault move) + 1", or 25 inches! Odds are pretty good that you won't get that far in the time you have to do it in. So don't bother. Get seven inches away, and hope the combat ends on your opponent's turn.
Evalutate this each turn of combat too. If the math would indicate that the battle will end during your next assault phase, take the second squad, who aren't able to get far enough away to avoid being assault, and counter-assault with them. They're going to end up in combat anyway at that point, so you might as well get the charge move and attack benefit.
If you run too far away, you lose this option.
You'll notice this constant theme in all these tactics. They all depend on having another unit nearby. Any defense-oriented squad (like a firebase) should never be expected to operate without support. To do so is to invite your opponent to assault them and take their victory points without much risk.