This is unlocked after completing Level 28 in Arena 3.
It's a balloon popping stage requiring 40 balloons over 3 traps (10,10,20).
The stage is designed like Donkey Kong, you'll be avoiding boulders by jumping over and ducking under them. The first two 10 sets require climbing the ledges against reds and bouncing yellows respectively - run back to the bottom quickly after getting to the top. The 20 set requires getting to the top, then back down again against bouncing yellows.
Tip: When jumping up the ledges, let the boulder go over the edge first, then use double jump from the raised platform to jump underneath the falling boulder to the ledge above.