You can totally stunlock it with Pierce 3 as well. Pierce 3 on the chest in rage mode does STUPID damage.You do way more damage with LunaXeno LBG while using elemental ammo, enough to stun lock him by targeting his chest while in rage mode which makes the fight so much easier. I already used affinity augment on my LBG but I can see health augment being a huge help in this fight.
I think most everything that hits the chest in rage mode can stunlock it.