Below are three great places to accumulate massive amounts of experience, thereby leveling up your characters more rapidly than usual.

Scorching Wind (7-1) - The whole field has a +100% experience bonus attached to it, and with tons of Zombies there, you'll find leveling up to be rather easy here.

Forest of the Dead (11-1) - Another field with a +100% experience bonus attached to it, Mages and Pumpkins galore to kill.

Cave of Ordeals 3 - A field with +50% experience bonuses attached, but there are tons of enemies here, and they're all bunched together, making for rather easy kills.