Corborus really shouldn't be the problem, your tank was the problem
I've downed Corborus in 12 seconds @ 220370.3 dps (in heroic ofc) on 5 epic-geared mages.

For things like spray poison, use repeater. Have your mages stand around same spot while dpsing, then when you get green stuff on you, just strafe left or right away from it and your all toons will do the same with repeater.