I'm wondering what levels you all are playing at when having these problems; when playing as Grom?
I play all games at Hard/Hard. (start there anyway)
VH/VH here.
I wonder if not dealing with Brettonia quickly enough could be part of the problem? Their early game armies are pretty weak, but that changes once they start spamming high-tier cavalry.
Personally, I didn't think it was the hardest campaign ever. And some of the stuff he can get is utterly ridiculous (Goblin Fanatics with explosive arrows and regeneration - basically the ultimate guerrilla warrior).