Rat Pack

each time Lumbering Oaf uses his charge ability on a player, random number of Mine Rats spawn around the lumber pile which the player was smashed into. Keep the boss alive until you kill at least 20 of them (4-5 charges should be enough IMO).

Here is what you do at 90:

You need Lumbering Oaf ALIVE - he is the one who spawns the rats!

Only use Auto attacks until you have killed enough rats. Should you have problems with keeping him alive, you can run just outside the door where you came from, and the encounter will reset.

NOTE that you dont need 20 rats killed in a single fight - you can reset the fight infinitely until you have it.

Actually this achievement can be completed over several visits to deadmines, just like the Iron Dwarf, Medium Rare achievement in Ulduar. Did this today with a group, got 9 rats (and achievement, since I've been killing rats ocassionally in earlier visits to Deadmines) after first Charge, 17 after second, and 25 after third.