At close contact range, it might be better to physically take him down, and if he's still fighting, shoot him in the face.
Gas masks, last I checked, don't stop most bullets.
This would require a rear or flank attack, initially.
Might sound cold, but if he was armored over torso, legs, neck, and head, it would be (IMO) very reasonable.
(Edit: Common tactic against armored knights in earlier days: knock them off their feet, then cut their throats, or stab them in the armpits or groins with daggers. Same concept applies.)
Last edited by MLeake; July 22, 2012 at 12:47 AM.