Fierce Stance on Grima?

It's good, but probably not the best. Grima's attack is already sky high, so you should probably focus on his defenses. Close/Distant Defense are arguably the best A skills for him.

I'd save the Zelgius given that he's still one of the best red units in the game despite the -atk bane, and Fierce stance isn't a skill worth giving up an S-tier unit for in general. Fury is a good budget option for any unit with decent bulk to get you through until you find a good fodder for things like close def / distant def / steady breath

I had an extra zelgius with the same bane and decided to use Fierce stance on my Grima. It works like a charm and if and only if you have a Hardin you have to fodder, Bold fighter works well with Fierce stance. Use Bonfire with Grima for more kills. Brazen Def/Res works well with him also. The boost from that saves him from A LOT of potential death. Brazen Def/Res, Bold Fighter and whatever C slot you want plus Bonfire as a special works wonders once he's in that threshold.