Board Thread:Questions and Answers/@comment-27023030-20150929050910/@comment-7326455-20150930022644

Oh, please. If you think that level's hard to deal with, then you may as well just quit now. The level six upgrade requires you to collect six hundred crystals, as well as waiting for the Air region to unlock.

It's not that hard to get from level seven to level nine. Just keep feeding monsters.