(adsbygoogle = window.adsbygoogle || []).push({}); First off, people have posted SIMILAR grind guides, but none were as effective as this method. Note that this is for IDLING, not ACTIVE grinding.

Requirements:
1 Character w/ game beaten who can SOLO painter
0-3 Characters who you would like to level up. (Note, the more characters the faster)
1-4 Controllers (Must have 1 controller per character)
BiPolar Bear (Found at end of ice world using horn)

Steps:
1. Put BiPolar bear on ALL characters
2. Have all of your characters in a game, preferrably local, and go to wizard castle interior
3. From wizard castle interior, beat all of the minions and go into the painter room
4. Hurt the painter (50 Combo and indutrialist work great) until the Painter has ~25% health
5. If done correctly the painter should now begin walking around and spamming paintings
6. Move all your characters onto the very bottom of the screen,and one by one kill a couple paintings and hit the painter a few times, using the basic melee. Do this for ALL characters AFTER he stops using the canvas, otherwise the character will receive no XP (Note, the BiPolar bears should be attacking the paintings)
7. Move all characters into the bottom left corner facing the SAME direction
8. Go to bed, and wake up many levels higher
For myself, gain was 1 -> 13, 60 -> 63, this was after a short period of less than 12 hours. Leaving the ps3 on for extended periods of time is not detrimental (to the best of my knowledge), however the fan can get annoying if you have the old ps3.
9. Once done grinding, make sure to EXIT TO MAP or the XP will not stick, after it saves, you can turn off the ps3.

Explanation:
BiPolar bear automatically attacks enemies anad kills them when they are below a certain health. We can abuse this because the painters paintings automatically are killed by BiPolar bear. The reason the characters are on the bottom is so they cannot be hit, and they face the same direction so BiPolar bear is on the same side. Having multiple BiPolar bears causes the bears to attack the same target simultaneously, and hit more often than can be compensated by using giraffee, i.e.
50xp + 10%= 55xp, 50xp x 3 = 150xp

Important: Note that BiPolar bear does NOT kill all of the paintings and will NEVER be able to. Even with multiple bears and even if they stagger, they will still not kill all of the paintings. Also note that BiPolar bear will sometimes stagger a little bit, despite the setup being perfect. Don't worry, they will either sync back up after some time, or target different paintings, before eventually automatically resyncing.

Active grinding:
Active grinding can be used for when you want to go up a couple levels quickly, i.e. 78 -> 79, 98 ->99, 48 -> 50
I find that the best way to actively grind a single character is play industrial castle. Reason being that the boomerang glitch still holds on the metal spikes that shoot elecrical orbs, as well as the central 'stomper'. I was able to get from 97 to 99 playing through the level twice. At lower levels it would be possible to level up multiple times. Note, this time giraffe helps a lot.
Obviously, insane mode is the best option.

Reserved for expansion