Unity of the Elements

The Devil is more fun; you just need to get lucky. I haven't tried bringing him with a large army – ghosts are better for that.

It would also be a good idea to transform Necropolis structures into Academy structures along with the castles. The army would accumulate a little faster, and you wouldn't have to wait as long.
Although, on the other hand, half of my army at the end of the game consisted of necromancers. And I didn't even think about gargoyles in the battle with the boss; I must have forgotten about them somewhere before that. And I didn't accumulate an army, either; I fought off almost the entire game with just titans and various minor units, which was fine because I had regeneration (a pretty good one, restoring 3-4 titans) and a powerful spell.