Ultimate Chicken Horse

The following achievements unlock by entering a level with a question mark on it and collecting the mystery box in the level The reward is completely random and can give you either a piece of costume, a new map, or a character.
You should start working on these achievements from the very start, and always enter a level with a question mark on it. If you end up earning all other achievements before unlocking all costumes, keep starting a level and quitting until you see a question mark appear.

This achievement is slightly buggy and seems to work only for the host.
Start a game with shortest number of rounds with two players, and keep dying until you reach sudden death. then keep falling to your death until the timer runs out.