Alright. Something is weird here. I've checked out video tutorials and read how everyone beat him but it isn't working for me. I've broken his body apart twice and killed the arm without his head and his legs. I need to break him apart one more time but every time I stun him his chest only glows red or it's yellow for an instant after I pump a million rounds into him and then he breaks out of his stun so I can't blow up a bomb on him.
I'm playing on Hard, by the way. What gives?! I've been fighting him for like 50 minutes now. He can't even hit me but I can't get through this last pattern. --- Banana Banana Banana Banana Terracotta Banana Terracotta Terracotta Pie! Banana Banana Banana Banana Terracotta Banana Terracotta Terracotta Pie!
I had the exact same problem on the same difficulty. My theory is that so much damage must be done in those few seconds that you stun him. Therefore, if you didnt find enough red diamonds to upgrade damage on the shotgun and pistol then you just get screwed like me and you. Its **** and needs an update.
I had the same problem until I stopped waiting for the moment to use the grenade launcher, and just attacked his head normally. That seemed to wear him down enough to where I could blow him apart. --- http://img638.imageshack.us/img638/6429/85350992.jpg
Not sure about the hard difficulty, but on Demon Hunter (Normal) difficulty, you just have to keep pumping rounds from The Dentist to bring his separated body parts down. When his parts split up, dash right towards the one which is glowing red and stay on it..