well i haven't killed any of these but from the info i have the reason grobb hide cowl is so rare is because it actually drops off bruisers :-P but could drop off both for all i know only killed bruisers and yeah it was very rare hehe
A full 2K bolt takes ~20% of their health so they have around 10-12K HP. They hit for 150ish. Pure melee nothing else. Normal resists for their level. Exp is > 1% AA with 90% going into AA.
The Grobb Hide Cowl is an ultra rare drop. Have had one dropping from more then 100 kills.
I'll post a screener of the stats later. Very nice BL cap in my opinion.
I started with a full mana bar (243 WIS with KEI) and had about 5% left at the end. It took three stackings of "Winged Death/Drifting Death/Breath of Ro/Drones of Doom" to finish him off as well as about 5 roots.
Granted I got 2% AA exp for the kill, it still didn't seem quite worth the effort. I can get a better mana/exp ration from killing blues/lt blues in Dulak.