Board Thread:Questions and Answers/@comment-4775209-20131123170010/@comment-3153244-20131124025426

Alternatively, you can keep randomising until you get a Random Challenge such as 20 Ceramic Bloons on the Wall (if they haven't patched it on iOS) or Third Time's A Charm and play until you reach 2000,000 pops.

EDIT: Yeah, I agree with Anon #1. Go Freeplay.