So this game never had achievements till recently making it an absolute cheese factory for rare achievements since all the people that had played the game are unlikely to return to it. Also not a best seller either, so there wont be a lot of new players getting the achievements.
Anyway here is your guide to get 100%:

Headbutt Zero key on character select menu. Secret character unlocked, secret character is OP. Kill everything, and explore every floor. Pick up every item, open every chest, and drink from the pool.

After winning with the secret character play as the main three to get their achievements.

Done. EZ. Uninstall.