Where am I going wrong with Haderax?

Just can’t seem to kill Haderax.

L80 Gunzerker - I have a Conference Call, Toothpick & Bee but no matter what I do, I can’t seem to do significant damage to him. Even if I get up on the ledge, it’s difficult to do enough damage to the crit on his head to stun him and, even when I do, I can’t get round to shoot his mouth quickly enough.

I’ve reviewed loads of videos and they all make it look stupidly easy - where am I going wrong?