If you're above level 40, you could do the Fool's Gold quest and the Polluted Rock quest repeatedly at leprechauns. If you go finish the quest each time you get all the rocks, the experience really starts to add up. I always take my rocks to Cassandra so I can get an extra buff while I'm killing the leprechauns.
So, Leprechauns <Lvl 40+, Exp: ok, Mesos: Godly>