Originally Posted by waxc3
To grind XP you can play a level that is difficult where there are many many enemies. Kill kill kill and die before the next checkpoint. You keep all XP earned before your death.
I learned the hard way dying over and over on a level in the second world and noticed I leveled up, died and kept the level increase.
Playing any of the survivals on hard or insane will be the best way to lvl up. You will also earn more money.