Iaido Class?

[SandyAndy]

You could make it go the other way too, rather than being the first shot, Iaido grants the fighter a reaction attack the first time they are attacked in combat. If they hit its an auto-crit.

Yeah, this is what I was thinking. Iaido is less about being a really powerful warrior and more about being a really alert warrior. I think maybe adding wisdom or perception to the "to-hit" modifier and auto-crit would be a perfect way to represent it.