Online Challenge: Island Challenge
The Island Challenge has got 300 Levels with various rewards given to the player when certain levels are reached.
The team matchups seem to get exponentially harder the higher in level you get. Not confirmed, but it seems as if the stats of the enemy team you get matched with get boosted, and the amount of boost given increases more and more as you climb in level. I suspect that it may be in order to create the "challenge," as the name suggests, to help keep the level cap at 300, and to make it difficult to acquire those high-level items. It becomes very difficult to play after around stage 160, as the increase in enemy stats just becomes too difficult to match if you don't pick out/aren't provided a perfect team setup.