Yeah hard to put it into words other than dont stand in the slime... move your tank and spam IWT for your dps. I have my healer following my tank as well during that phase so everyone is moving.

My tank does roughly 2400 DPS and my DPS do like 1500 (backwards I know) and i kill him after one switch (i.e hes stone man, then goes to water dude, back to stone man, I kill stone man, water man comes out and i kill water dude...

Good luck.