"In Critical and baffling situations, it is always best to return
to first principle and simple action"
Reserves and/or escalation
Reserves are, as the name implies, units that you do not deploy at the beginning of the game, instead, holding them back to deploy in response to your opponent's moves. Escalation is an Omega-level rule that states that only infantry models and infiltrators may start on the table, everything else begins in reserve.
I've been reading a book about decisive battles in history. About half of them seem to have been won or lost by the correct or incorrect usage of reserve units. So, it's worth paying attention to them.
Why would you choose to start with some troops off the board? A large part of the answer to that question is based on the mission in question. Say, for instance, that you're the defender, and have to prevent the attacker from getting his troops into your deployment zone. If you deploy your forces all on the table, you're either going to only be defending part of your zone, or you'll be spread out. The attacker can take advantage of either of those situations. If you spread out, they can concentrate their force and overwhelm you at one point. If you bunch up, they can simply go to the part of the zone where you're not bunched.
That's where reserves come in. You can deploy small units spread out throughout your deployment zone initially, and hold the majority of your army in reserve. When you roll for them to come in, you can bring them in whatever location the attacker has concentrated their attack on.
Which units do you want to hold in reserve? In an escalation mission, you are limited in what is allowed to start on the board. But otherwise, the best reserves are those units that are mobile enough to get into the fighting whenever they show up. It is no good holding back foot-slogging infantry, because if they don't show up until turn 4, they're going to end up, at most, eighteen inches from where they started. Elites and Fast Attack choices make good reserves, because they have the capability to pack a significant counter-attack. Troops, on the other hand, are better deployed at the begining of the game, as they're going to have to hold off the enemies early attacks.
"Frederick [the Great of Prussia] liked to say that three men
behind the enemy were worth fifty in front of him."
Deep-strike is a specific type of reserve move. It is one of those things that can win or lose games based on the roll of a die. When it goes well, it can go really well. You can deliver your anti-tank squad right behind the enemy tank, and explode its rear armour without much hassle. When it goes wrong, it can go horribly wrong. You can lose hundreds of points due to a poor scatter. Your troops can be left out in the open, waiting to be shot or charged. They can, potentially, never show up at all.
After playing a few games with deep-strike - both for and against, I have come to the following conclusions - your milage may vary.
First, Deep-Strike at most one unit. The odds that multiple units will arrive separately and unable to bring significant mass to bear is too high otherwise. This suggestion applies to true deep-striking only, not to things that bear a resemblance to deep-striking, like drop-pods, or daemon summoning. Sure, there are some fancy all-deep-strike armies, but they're special cases.
Second, have a purpose for your deep-striking unit. The traditional use is the specialized anti-tank squad who will arrive behind a tank and make their points back with that one kill. If they don't accomplish anything else for the rest of the game, they've still accomplished their task. Expect that you will lose any unit that you deep-strike away from the support of the rest of your army.
Finally, make sure your deep-strikers will survive to do their job. Pay attention to possible scatter errors. Sometimes, you will have to take a calculated risk in order to get the squad where it needs to be. But, you can minimize that risk. Try to stay away from table-edges - deep-striking on a table edge has a 33% chance of losing the squad. Stay well away from table corners - they have a nearly 50% chance of losing your squad. Make sure that an assault-oriented deep-strike unit will have enough armour, or wounds to survive whatever guns they're in range of for the turn before they're allowed to assault.
Dropping 500 points of grey knight terminators in position behind cover, but able to assault the next turn will probably win a game. Losing those 500 points when they land on a random ork grot will certainly lose it.
How do you counteract deep-strikers? The easiest way is to minimize the number of locations that they can land successfully. If you know that your opponent plans to deep-strike anti-tank troops, keeping a unit of troops near the tank can create a deterent, as it increases the odds that the scatter will end up losing the unit among your models. When you spread out in this fashion, you will want to utilize the full two-inches allowed between all of your models, so that the unit occupies as much space as possible. If you keep your tank close to the edge of the board, then deep-striking near it carries a much higher risk of losing the unit than if it can be approached safely from any side.
Infiltration is a special ability that can allow a unit enhanced early mobility. You get to deploy infiltrators after all other units are on the board. This can give you an advantage in claiming objectives, or setting up an attack. However, it can also lead to early destruction of your unit.
Infiltrated units are likely to be deployed well advanced from any support. If they don't get the first turn, it can be easy for your opponent to surround and destroy them before they accomplish anything. When setting up your infiltrators, you should always assume that you will go second, and err on the side of caution when deploying your infiltrators. As with deep-striking troops, it is best if you have a mission for them in mind, before you deploy them. Losing your infiltrators early may be acceptable if they are able to disable an enemy tank, or tie up a fast assault unit for a couple of turns.
How do you counteract infiltration? The rules for infiltration state that you can infiltrate within 12 inches, if none of your opponent's models can draw a line of sight to you. So, the best way to keep enemy models further away than 12 inches is to think about possible infiltration locations as you deploy your forces, and make sure that someone in your army can draw a line of sight to those locations. It doesn't have to be the unit that is closest to the location, as long as someone can see it. This will force the infiltrators to start at least 18 inches from your lines, which can reduce their effectiveness. If your opponent is foolish and deploys infiltrators without support, you can take advantage of it and attack their position with local superiority, suffering minimal casualties in return.