@shusi95
I assume it's per activation, but this also means if he can't finish off or back away after 2 seconds, he's vulnerable again until he activates his ability again. And there's definitely a limit or cooldown to it.
And yeah, like the other poster said, it's a useless ability for anyone who isn't a physically strong monster in the first place. Imagine if the protagonist had it - what the hell would he do with it? Survive 2 seconds, assuming he can react to an ability or even weapon targeting him, then get slaughtered.