I just looked at the image above ... wow, did you spend 400 gems empowering your Devourer? You must be rich!
I didn't realize you had an accompanying army. I agree that's the best way for the devourer to be used. I still manage to get mine killed before it retreats though
. When your army takes 50% HP casualties, that causes a morale check every combat turn. If you've got something big and not-so-useful, like a feebleminded Monstrum, you could have it Guard Commander something at the back of your army, to keep your HP totals up.
But aren't you worried your Devourer is going to get Soul Slayed or Petrified? That's an awful lot of gems invested in that MR18 unit. And he's got enough HP that he's probably going to be targeted with whatever nasty spells are flying about.