Here is the full list of all 9 ChestMate Playtest achievements.
Kill a five eyes jellyfish
Kill 100 skeletons
Kill 2 enemies with one cannon ball
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
Kill 10 enemies with one explosive barrel
Kill a pink starfish with a green short