As a ranger, here was my strategy to kill the 4K of this achievement that remained.
Insure you do some version of /hidecorpse. Stand in the very NE corner of the rat room. F8 and autofire with your bow. I will able to kill a bit over 100 rats in 3 minutes. I also setup a macro, though with an occasional UI glitch (a corpse remain), this didn't work so well after a relatively short period of time. I would say I spent around 5 and half to 6 hours, with lots of interruptions to complete this achievement.