Playing Your Game
"Victorious warriors win first and then go to war, while defeated warriors
go to war first and then seek to win."
Hopefully, by now, we have some good tools to use in determining what our armies are capable of, and what they will have problems doing. We should be able to classify our army, and by doing so, know how to design a game plan that will emphasize its strengths, while minimizing its weaknesses.
In any game, one of the easiest ways to score a victory is to force your opponent to play to your game plan, rather than allowing yourself to be tricked into playing into their plan. It is important to remember the basics of your game plan at all stages of the game, from deployment through the end of the last turn. Note, this doesn't mean you should be inflexible. In fact, what you will do if things don't work out as expected should be part of your gameplan. But it does mean that you shouldn't let your opponent dictate what sort of game will be played, especially not if they get a lucky shot and kill your tank on the first turn.
Adjust Your Ends to Your Means
When you set out your plan, you need to consider, how, exactly, do you plan to win the game. If there are objectives, which units can you rely on to hold those objectives? If there are victory points, how can you score the most while giving up the least?
Knowing what you believe you can achieve is important when it comes to making your plans. You need to realistically assess your units weaknesses as they relate to the mission at hand. Maybe your army is not realistically fast enough to get to the objective. When this happens, rather than make a futile attempt at something that you know in advance won't work, you need to adjust your plan. Denying the opponent the objective can be just as successful as seizing it yourself. If you know that you cannot easily score victory points against your opponent, without giving up points of your own, then seek to avoid the fight and play for a draw. By keeping a the expectations of your army realistic, you can make sure that you play towards end results that you have a good chance of accomplishing.
The Real Strength of Assault
There are very few things that you can do in Warhammer 40k that compel your opponent to take actions that they may not want to. Key among these is assault. And, this is the true strength of the assault phase. Simply by getting your models into contact with your opponent's, they are forced to join the combat, and remain involved in the combat until it is finished. During this time, they are unable to fire their guns, move, and may well block lines of sight that the rest of the opponent's army was depending on.
Moreso than any other part of the game, the assault phase is where the assaulting player can dictate what will happen. The most that a shooting army can force the opponent to do is move their units into cover. With target priority rules, they cannot even prevent you from shooting at a unit that's behind another unit. The assault army, however, can not just kill models, it can render them useless while they're dying.