The big ooze doesn't hurt very much after the first merge. Every time it merges it gets a stacking damage buff. You can tank it a bit to build aggro when it first merges into the big ooze... just be careful.
I usually vigilance the MT and taunt spam it. I was having trouble as well until I tanked it a bit (like 3-4 gcd's) when it first merges. After i tank it a bit I intervene someone to get out of range.