Hard to find the good way to make them stronger but not too much...
I think you have to create an expensive item for each boss who disable an ability of the boss. Without this item you just can't kill them.
I don't know if it's possible...
Or you can just increase the hero max lvl if it is a second game.
I hope I help you. Good luck, it's very hard to balance a campaign suck like that.
Hey, based on my observation,
Each boss's spells were made with triggers.
If you still want Aero to create an item specialized to defeat that boss,
It would require MORE triggers.
And that would DEFINITELY delay the progress of TCO.
Come on