-In tactical combat, the AI is attracted to your hero. You can use your hero to lure enemy units into a killing zone of ranged fire. For non-hero battles, place a ranged unit out front to become the lure. Retreat your siege engines from combat as soon as the wall is breached. They are too expensive to be destroyed by token infantry attacks.
-Migrate friendly citizens to all cities you conquer unless you are in a hurry or need gold right away, in which case, you should just loot. Never raze conquered cities, unless it is a victory condition.