Star Reef - 6

Strategy
Warning: Don't use the Rage Chili on the first wave. Use it to Bomb at the second wave (once only) and for Blue Bird for the first time the Rage Chili is full at wave 3 or when Frankenpig is die. If the Rage Chili is full for the second or third time but Frankenpig is still alive, use it for Matilda.(unlimited)

Choose Matilda in Druid, Bomb in Cannoneer and The Blues in Rogues. In every waves, attack every turn with Bomb and The Blues, then, heal with Matilda if the birds' health is not full, otherwise, attack with her. Repeat this over and over until you win or lose.

In Summary
Frankenpig and Necromancer are the big threats, so you have to beat them fast!