I would definitely not recommend a pneumatic puncher this year as even two full cans of air will either (1) not have enough air to launch 22 triballs (2) not have enough air to get all the triballs over the barrier. However, if you are going down that route, here are some threads:
Pneumatic Catapult? (discussion on the merits of pneumatic launchers)
https://www.youtube.com/watch?v=tmmjL8r8-E4 Here is a creative idea
pneumatics help - #18 by FullMetalMentor (some more ideas)
Search the forum and youtube for “VEX Toss Up Pneumatic Catapult” and you’ll get quite a few ideas