A large part of it is probably their heat aura, tiring your troops and making them easier to crit. Depending on your pretender choice, you may have multiple options, but I'd guess if you are very far into the game, and they are very big, you won't have a lot of time to develop something new.
So summons who are immune to fire, magic items for melee commanders that give FR (hard for you to make, mostly), and heavy evocation barrages (if you have a pile of Sorceresses and/or Kings at your capital). Sometimes in SP if the comp gets unruly in what is still basically the "mid-game", your most viable option is a researcher rush to squash their main armies. It's unbelievable (when you first see it) what 20 battle mages spamming thunderbolt can do to 400 heavy infantry.
