To beat nightmare AI simply kill the wounded at your towers, the AI is pretty dumb still. If you play normally (farm etc...) you are eventually going to be out-leveled and out-itemed by the AI, and then no amount of AI stupidity is going to help you.
You can also try taking brillant bauble and buying the EXP citadel upgrade to outlevel the AI. (as long as you keep killing the wounded AI at your towers)
It's not really good practice because what works in nightmare does not help you against real plyaers in a fair game.