Here is the full list of all 15 Chest Mate achievements.
Finish the first level
Kill 1000 skeletons
Finish all levels in regular mode
Finish all levels in storm mode
Finish all levels in storm and classic mode with all your chests
Kill 2 enemies with one cannon ball
Kill 3 enemies with one cannon ball
Kill a five eyes jellyfish
Kill a pink starfish with a green short
Kill 10 enemies with one explosive barrel
Help your mate dig
Kill 100 enemies with fire
Kill 100 enemies with explosive barrels
Kill 50 ennemies with the scatterbarrel
Kill 100 enemies with automatic cannons