When Toth turns a greenish color you should stop dps and go to the healers. This is during the rock throwing phase. There should be red text that comes up and says something like "Toth Enrages". During this phase, the only reason to stay next to toth is if you are the tank or in hard mode if you have the yellow circle. When he does his aoe jumps while enraged, he can take out melee who aren't fully healed.
This is probably what is killing you. When red text flashes on the screen "Toth is going beserk!" or something similar, all melee dps should get away asap, since this means all of Toth's attacks deal double damage. Once he jumps the first time after beserk (which you should have dodged) jump back in.