In nightfall, in the sulforous wastes, stick on HM, grab your low level heroes, jump in the wurms, and go kill stuff, very fast leveling. I went out with just me and 3 heroes (each about lvl 10ish) and before i knew it, they were level 20.
Depends how good you are.
I normally just grab the nooby booby heroes and go VQ (or whatever floats your boat really) some stuff and they magically gain a few levels.