Im trying to kill Rais' demolisher in the pit on New Game+ and Nightmare, and he hasn't taken any damage.

I've broken a heavy rebar, crumbled rebar, claw hammer, and plain hammer on his chest. I'm sure this is possible, but I'm 80 something hits in and I really can't tell if I'm gonna be able to do this. Is there a hidden jutsu or technique im supposed to use to scratch the red bar at the top of the screen?