You will get to a part in Genosha after finishing off a battle with Cable with a prize wheel. After each spin make sure to shoot the balloons.
After the second spin make sure to shoot the cow.
Total items should be: 12 balloons (I believe), 1 cow.
I initially missed the first set of balloons after the first spin, then after getting to where you get thrown (third platform up) I jumped off, died and restart. Started me at the bottom, I spun the wheel, shot the balloons I missed and it popped.