Me and my friend found this out.

You have to get to champion to do this, you can try it in the lower difficulties but i doubt it will work as well,
go down to feydark forest, and before you enter the caves you have to fight a boss, i forget the name but that doesnt matter, its not hard to figure out, weaken him untel he has a sliver of life, you should be able to acomplish this with 2 players with atleast 1 level 20 resing if you die, then befor he dies create a new character, then deleate the one you are using, and kill him with your level 1, this is the difficult part, you either have to have one of the guards kill him, but be carefull because if he kills the guard you gain 5 levels, this is not what we want, either that or you will have to do it your self with a bow, or the hard way, you get a sertain distance before he turns around then you have to run up and get him from behind. once he is dead you gain 42 levels with 1 bubble to the next level. Enjoy!