The best way to kill Grobull the Transmuted is to first kill all the large netches that do not die when he drops. Once they are dead, you must group the small adds and kill them at the same time. When the small adds dies, they shoot a power beam at the boss and if enough of them hit at the same time, his shield goes down instantly. This method takes the fight from 15 minutes to 5 minutes.
Hail Sithis!