This is for filling the 4th stat meter.
As you might expect, XP gets harder the more you level up. Diminishing returns.
I played a lot of Horde, since you get XP when you die, and don't have to finish the level. The more controllers you have, the more of a buddy system bonus you get.
There are 5 areas to level up, and you have 4 characters. If you have a favorite, go ahead and level them up twice, but you don't have to. You can get the 5 achievements by spreading the love around. (That's probably obvious.)
I preferred the Pleasantville level, but that's me. I like the crane.