Online Challenge: Island Challenge
The Island Challenge exactly has got 300 Levels with two monsters, an ingredient and among other things as rewards.
Seems to get exponentially harder the higher in level you get. Not sure, but it seems as if the stats of the enemy teams you get matched with get boosted more and more as you climb the levels in order to create the "challenge" imposed by the name, and to help keep the level cap at 300, also making it difficult to acquire those high level items. 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.