Made it so instead of having shots pass through him when he's invincible (achieved by having a body-area only on the one vulnerable animation), we instead make shots die upon touching his exterior, and make him flash white, when invincible (using default behavior available to all hittable inheritors).  This seems much more preferable; it indicates to players that yes, they are in fact correctly hitting the monster, but the monster just isn't being damaged by their shots.