How heavily to invest in +atk -def M!Grima?

It comes down to these 2 things
Do you need him?
Do you like him as a unit?
But he does get pretty ridiculous with that set up.

Well, you certainly can't win them all with IVs and 32 def isn't awful. The good thing is, +ATK is his best boon. Unless he is on a banner again soon, there is no really idea when he will be available again. I say go through with it because Grima is a very special unit. Worse case scenario, if you pull one with better IVs, you can simply merge the units and keep all skills. Nothing is lost.

EDIT: Also, unless Grima will be on an armor team with a unit that has armor march, stick with Vengeful Fighter. Bold Fighter requires you to initiate in which most armors are attacked first before they can initate.

Steady breath will help patch on his defense which makes vengeful fighter even more logical of a choice.

If you care about a high tier in the arena, grima (and hardin) are your tickets there. And that means giving them a skill that requires the most SP in each slot.
Hardin is an animal, and this site says a speed boon will actually help him deny a few follow-ups. When brazen def/res activates, his resistance bane won't matter. Most importantly, because yours doesn't have a superbane (-hp or spd), he's in the highest BST bucket.

Long story short: build him and Grima. Give steady or radiant breath to Hardin. Grima can use a different skill in the A slot, since he wants to tank ranged attacks most of the time (I sacked my Oliver to give him mirror strike and more potency on player phase).

I use them on the same team and that's what got me to tier 20. Don't sack your Hardin because he's just as good, and for arena score might be a little better. But start investing heavily in both of them and they will dominate.