Here is the full list of all 9 ChestMate Playtest achievements.
Kill a pink starfish with a green short
Kill a five eyes jellyfish
Kill 10 enemies with one explosive barrel
Kill 2 enemies with one cannon ball
Kill 100 skeletons
Finish the first level
Finish 10 levels
Finish 10 levels in storm mode
Finish 10 levels in storm and classic mode with all your score