I've been playing AI's a bit ( since I can't play humans except for that one day between updates, hint hint ).

Normal AI seem rather stupid. When low on HP they will often just walk away a bit and stand with their back to you, or walk back and forth inside your range and just be killed off easily.

Hard and Nightmare are actually hard to beat. They will escape in time, take haven behind their towers rather than in front of them, give good chase when you are in red, try to interrupt castings and go for a kill with bat teleport or pounce etc.

However, they also start the game with more cash. They will appear at start with siege gunners, potions and plenty teleport scrolls, sometims even with a Finger of God from get go. I don't know the exact settings.

There should be a level of strong AI behavior but equal conditions.