You have to kite him around and kill the worms that spawn. The worms drop pools of acid that he absorbs by walking near them. Kite him through those pits of acid and as they stack absorbing more and more of his shields / health will chip away. It takes a while and is best to do with a team of 2-4, I'd say. Sometimes, you absorb the pools of acid, at that point you either have a siren in your party with res or you die. It doesn't seem to want to go away even after death, so if you happen to absorb the acid, it might be a downhill battle from there. Try to make sure to kite him over the acid pools quickly though, as more worms can spawn and more pools can form, causing the ground to be covered in acid and constantly damaging you.
If you have a Flame of the Firehawk shield and a Rubi pistol, those can give you enough health regeneration to make it so that you are pretty much immortal as long as there is an acid pool to damage you.
See, Rubi heals you for 12% of any elemental damage you cause. Flame of the Firehawk shield constantly releases some of the most powerful Fire novas in the game so long as it doesn't start recharging. The constant damage from the acid will prevent any recharge attempts, for infinite novas, and the novas will be healing you to full health so long as they are hitting Gee or a Worm, or both.
I figure that you will need a Level 50 version of the shield for this trick, but that isn't a big issue, since it's a Quest reward from the end of Lilith's Cult Following quest chain.
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