Do You Want to Hit A Snowman
A straightforward side-objective situated on a snowy slope outside Manhattan.
Which is not to say easy. The uneven terrain can make a lot of distance weapons unpredicatable. Your best bet is to use Hulk, Man-Thing, Fing-Fang-Foom, or any other hero with the trampling ability, and just cannon through these snow goons.
Getting 20 under the time limit unlocks Hit-Monkey for purchase.