Just boxed it and beat it on second try with:
SK, ENC, CLE, WIZ, WAR (dps only... no tanking), MAGE
The first time it was just shitty because I failed to click the last item on Grummus to drop his shield, but I didn't notice my SK wasn't meleeing it until it was too late.
Second time I took the four mobs down in the order described on eqresource and alla's, and clicked the clickies in the order listed there. Shield dropped this time and I zerged it down basically ignoring the barrels (more or less) and letting the cleric heal through. It was not pretty, and two toons did die, but Grummus died in the end.
Sorry that this isn't chalk full of info, but the threads here on it were helpful. When I'm not at work I'll try to find a link for you. I think some of the Grummus info is buried in an Anashti Sul group mission thread.
edit: all toons are in TBM group gear, and the SK has a Blue Tower and TDS chase knight 1 hander. No raid gear, no fancy items, etc. SK, ENC, CLE, WIZ are max AA, the WAR and MAG are a couple thousand AA short.