This advice is questionable. I say that because there are a ot of complicating factors (including dealing more damage with greater unarmed skill), and I haven't actually tried out increasing unarmed in an effort to not kill enemies.
That said, were Grace to increase her unarmed skill, it would unlock two special moves that could potentially knock down enemies. If not augmented with unarmed weaponry (special gloves) and no perks, I'm pretty sure she could do this without killing the sort of opponents she would be desperate enough to try those moves on.
A better plan, and one that Tedd might have suggested if it were simpler to explain in a comic, would be to complete missions that result in NPCs teaching Grace special moves. In particular, there's a quest that rewards learning "The Ranger Takedown", which has the knockdown effect Grace wants without needing to increase the power of her unarmed attacks.
It's a lot simpler for comic purposes, however, to just suggest getting better at unarmed combat, and it makes sense in the context of Grace not wanting to shoot anything.
In other words, please don't yell at me, Fallout: New Vegas experts.