From your post, the only context that I was able to figure out, was a timid LO Greninja against a 252 HP Rotom (so likely not a scarfed one). Regardless, your assumption that scarf Rotom can OHKO Greninja with any electric move is only valid if said Rotom is switching into a predicted water move. Otherwise, Greninja will always lose its water typing, by the time Rotom comes in. Finally, and for completeness, scarf-Rotom takes 56.1 - 66.5% from LO Greninja's dark pulse, so it's a risky switch in, to say the least.