I dunno if this should be considered an exploit or a guide...but I don't think Blizz intended it to work this way. Anways, we all know about the Test of Endurance quest in 1k needles; the one where you break some boxes in the cave and fight 4 waves of harpies.
I originally tried breaking more than one box at a time to see if I'd get multiple waves, no luck. I then tried breaking one box, killing a wave, and breaking another, then killing a wave and breaking another. At this point I had three waves with 2-3 harpies each going at once, so 7-9 harpies total. I was in a group at the time to do the quest, so we handled em real quick. In any case, we kept doing this for about an hour and were getting amazing xp.
So yeah, grab a group of about 4 around lvl 26-30, preferably a pally tank for aoe tanking, a healer, and two dps and you can grind mobs pretty much non-stop. The harpies hits are light and the silence doesn't really matter if your healer stands in the back. We did this for about two hours and I got a level and a half (I was 27 when we started). Make sure to have your healer break the boxes as they respawn so the waves dont stop.
Note: I searched for "test of endurance", "harpy", "harpy grinding", and "harpies" before posting; didn't get anything.