So you and your buddy just got your first character to level 50, and you want to level up another one but you don't want to go through the first few missions from 0. You wanna try out that fuckin' berserk mode amirite?

Well, with the help of a high-level friend, you can power level a character up to levels 13-15 in a matter of minutes. All you need is a high level character that's on the second play-through at least as far as the Awesome Rakk Hive. Have your friend create a new game on the 2nd playthrough and you join it on your brand new character. When you enter the game, it'll whine about how far your friend is ahead of you and how you won't be rewarded for any of the quests, but that's okay - disregard that.

Your friend warps to Trash Coast and races around to the left to fight the beast. If your friend kills any Rakks in the process you'll probably be at level 4 or so in the process, and once the hive dies you should be anywhere between 13 and 15 depending on how many Rakks were killed.

Rinse and repeat for three new classes, then return the favor for your friend, and you should both get all four class's action trophies in about a half hour.

This method is effective power-levelling up to about level 40, where it finally slows down to below one level per kill.

Enjoy.