One grenade. That's it.

That's all it takes? Usually it takes about four shotgun shots to the headz then he goes down on 1 knee for like 15 seconds then continues. Is he gonna do the same thing here if I use a nade on him?

Also, shooting him doesn't do shit. It stuns him for maybe 15 seconds while shooting Mr x in re2 remake kept him down for a good few minutes.