In my experience, my 17 years of wargaming experience, I have found that victory favors the aggressor. There are instances where a turtle seems the answer but ultimately they will lose to a more aggressive player as the aggressive player continue to develope new strategy. Look at it this way. If an opponent is forced to continuously respond to your assault then they have little ability to force a counter attack, will make a rash decision which and allow you to dominate them.
Even though your game plan is to simply run the Nightmare up and kill their caster (It always works for me) it is always wise to have 2 - 3 back up contingencies available. This way should your primary win condition fall short you will not require as much energy to regroup. Also, this let's you remain on the offensive (see "Be Aggressive.")
The best part about war gaming is the human interaction. There's nothing better than a few chums gathered together for a little brain bash'n. Even at a tournament the atmosphere and energy level should be fairly relaxed. Keep it this way by not trying to deceive your opponent. When they ask you for stat information don't lie. Don't erase wounds and especially don't try to move your figs farther than their alloted movement. Not only will dishonesty result in a quick sock-jacking, you'll soon find it difficult to get a game. I've seen this first hand as continual problem players fade from the local game group.
....more to come....