This is a Game Mode, which you need a LOT of Area of Effect towers (Bomb Shooters, Bloonjitsu Ninjas, etc.) to complete. Unlike other Game Modes, this one has a freeplay after Round 50. It's automatically set on Medium. The challenge is: All bloons and blimps will come in RUSHES, rather than random. Yup, blimps will come in rushes too. Just to have an idea that how blimp rushes are, think in MOAB Madness. Done, that's the rush of all blimps.