Sorry, i think a better title would be "experience tips"
There are three that i know of and have used
1. Go to the industrial castle and use your boomerang to defeat the end boss. If you throw it behind and wait for it to get caught on the way back, you get exp for each hit (should be 1)
2. Go to the painter boss, and using the bi-polar bear. After you damage him enough, his creations will come out from the sides. Rubber band your left stick up so your character keeps moving upward, and put the controller down. Each hit your bear does is exp, but it will take long so go sleep and wake up 8 more hours later (slow)
A similar but faster way is to kill this painter boss, collect your money and either go left or right. By re-entering this room, you can keep killing him, and earning money each time.
3. You can go to the map location right after the Barbarian boss. Near the end, before you ride the deer, you will fight a troll (black monster). Kill its minions, and you will get exp. Also, its an unlimited spawn as long as the troll is alive
Any questions, please post and PM me, so i may hit any questions