You don't need all achievements (those of creatures that are not jet in game) to get that outfit.
You need at least 25 completed achievements (in 0.5.4.2) to unlock skin 5 and there is 31 unlockable achievements in this version.
Edit to clarify: Naked skin is 4th skin in the list. But in game code (achievements) it is marked as 5th (most likely legacy thing because in 0.2.1 it was 5th skin, or it will be moved when last skin gets released). So my apologies for confusion.