Playing as the Legions (Disciples 2).

Yes, unfortunately, the AI is as dumb as in almost all fantasy turn-based strategies.

Not just in them! Legends and anecdotes about the AI's stupidity in Oblivion have been circulating for a long time.
It would probably look amazing to have a hero with two Tiamats. But I haven't tried it. First, I was afraid of Tiamats because their appearance is scarier than an atomic war, and then I decided that an extra Imp and Incubus would be better than two two-headed creatures.