The place i have used last year and just started there again is Naxx. Get to Kel'thuzad and engage the fight. You will be killing the skeletons and md'ing the rest to your pet. When Kel starts attacking or you get overwhelmed just feign death and it will reset. Takes a few seconds for him to respawn but the amount of mobs and their rate of respawn is very nice.
Could work for other classes but you would need to die and res over and over.
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