Don't poison dance on the collosus - it's an awful tactic. Put down a nature's resist totem. Spread your guys out against the wall/railing opposite the colossus so characters that get charged by the poison elemental don't get knocked al over. Pull. DPS. When he goes and spits poison clouds out, just chain heal. The poison does almost nothing as a matter of fact when you are spread out - like 400 a second or something trivial. Just chain heal (or even better aoe heal if you have a druid or priest) and dps. Don't move at all the whole fight.