which a skill should i give me fallen robin?

The most dangerous Grima combo is Steady Breath, Bold Fighter, Quick Riposte seal. If you don't have Bold Fighter to give him, I'd recommend holding off Steady Breath. His default Vengeful Fighter already includes a charge accelerator, so Steady Breath will be half-wasted.

The spd boon is actually very usable. Get someone to give him a hone armour, and you're at 33 spd which is defensively useful. His statline will look like a speed-oriented Nowi's except with about 15 extra atk.

Assuming the hone armour (otherwise I think you're throwing away the potential of the spd boon), atk/def bond I think is probably the way to go if you want to one-shot people. In that case I would consider death blow if you want to do that on player phase and fury or brazen atk/def or even brazen atk/spd for dual phase. M.Grima with 40 speed? Atk in the upper 60s is enough to one-shot a lot of things, including most green mages, some blue armours, and squishy red things. That will even save you a bold fighter. If you're going the brazen route I would consider vantage. Forget about getting hit any more, building special charges, just blast them clean off the map before they even get to you.