Are you talking about the fight in the chapel?
Very light spoiler below

Me and my coop buddy just beat this for the first time last night. One hint I can give you is focus down the green guy as fast as possible. I used a summoned fire elemental to distract the others and just unloaded every thing i had on the slimer wanna-be. He accounts for the majority of the spike damage and dot damage in the fight. Get rid of him, have jahan cast rain, figure it out from there smile