Seemed to have some good luck with a previous Firefly quote, so why not? We're all set in Ahn'kahet, so let's pop over to Azjol-Nerub and lay waste to some bosses there.
Every achievement here is, in one way or another, a DPS race. If you've had difficulty with previous achievements in that vein or your DPS is still gearing up, you'll probably want to steer clear of Azjol for a while.
Watch Him Die
This is one of the few achievements in the game most efficiently done from a kamikaze perspective (Sarth-10 3D is the most notable of the other picks). Early attempts at this with two tanks, two DPS, and one healer failed miserably, mostly due to the combination of poisons, a Priest healer, and Watcher Narjil's Blinding Webs ability. I'll admit I tried that before anything good dropped from Naxx, so it may just have been a gear issue at the time, but:
a). If you have very good DPS, and:
b). If you're willing to suffer a repair bill
-- you can probably get this achievement in one or two tries without worrying about keeping all three Watchers tanked, bringing an off-tank, or risking a DPS having to kite all three to the entrance.