When you engage him, you have 10 sec. before he casts transformation.
Less than 50% health will cause him to cast it ever 5 sec.
Bring him to 51%, let him cast in, then nuke him.
"Humility defeats pride, Master Yang has preached. Pride defeats man"
Happy to say that while I was on my weekly Naxx solo clear yesterday I swung by and tried and finally got it! Thanks all for your help.
We're talking about this boss : http://www.wowhead.com/npc=29305/moorabi aren't we ???
Damn, 430k HP, it's nothing... You should be able to kill it with 3 GCD or less, I don't understand how you can miss this achievement; no need to interrupt anything at all, just kill it almost-instantly.