Hey guys, this is my first post here so go easy on me pl0x =D
I found out that probably the best way to do the Crittergeddon Achievement is just to enter the instance of non-heroic Gun'Drak (left hand entrance, while facing the keep) and kill the Snakes in the "Snake Pit" which you enter right before foghting the first boss.
[spoiler]
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Enter.
Kill 'am all.
Leave instance.
Reset.
Repeat.
So long,
Arland