Head to Crey's Folly find a Fire Tank, or better yet 3 or 4 of them.
Then do the following:
Herd as many Freaks as possible. I went from lvl 30 to lvl 40 in 7 days using this strategy.
I was getting, according to Hero Stats, up to 9000 exp per min at one point!
This works very well, power leveled half of my SG doing this