F!Grima +atk -hp or +spd - def?

I would say go +atk if you want more of a enemy phrase unit
Go +spd if you want a player phrase unit

Grima does not have a player phase unless you put Bold Fighter on him. Speed is his best dump stat. Atk, however, is his best boon because it is a super boon, so I would go with +atk.

I would hold off on merging just because both Vengeful Fighter and Ward Dragons are both extremely valuable skills. But you can merge if you want.

I'd go with +Atk -HP. That's her best IV, IMO.

In my experience, -Def makes her kind of squishy (I have +Res -Def) -- even with Dragonskin on. Remember that -Def is a superbane, too, so that's going to hit her bulk a little harder than -HP does.