Other A slot options for Grima(M)

Close def/distance def, Atk/def or Atk/res bond are four solid option for his A slot. too. In short : Any skill who upgrade his bulk are good for a build with vengefull fighter, even +3def/+3res are good for a "budget" build

My fave is Atk/def bond with aether for scoring, ultimately its preference and fodder, breath is the best because it doesnt "waste CDR" on either phase, so with BF you always get a 5 turn CD spec, but with VF it doesnt matter much, depends on IV, and team support. If he has a wave skill (atk/res/def) give him the counterpart to that, whether iitsAtk/res or Atk/def or whatever

M!Grima main weakness is his less than desirable defense, so close def/dis def is quite a good option for him. Steady is less good because it can't stack with vengeful, but if you wanted to make him into the truly unbeatable then swap vengeful with bold fighter then put death blow to his a slot and see how much fodder he destroy.

Others have suggested good ideas. I would also consider brazen atk/def, brazen atk/res, and brazen def/res.