Hit it till it's hp is 0? :')
But for real, it doesn't have an invuln but is under constant rooted state (?) so you can't move it. It goes invis on the first turn but it's quite easy to find where it is since the AI is quite aggressive. I'd suggest keeping the rest of the mob back and blitzing No-Face first. Also, be careful of the purses of kamas that it summons, they have quite high lock but i'm not 100% sure what happens if you end in the glyph/next to them. Look up some YT vids, the french translation is "Sans Visage." Gl ^^