Here is the full list of all 17 Ghost Croquet achievements.
Kill the hero in every possible (ghost-controlled) way
Have 3 upgrades at a time
Kill the hero with an egg that flew a long distance in a straight line
As a slime, land on hero’s head
Deflect an egg into the bird that dropped it
As a ghost, kill a monster possessed by another ghost
Complete a level with a top hat
Beat a level killing every monster
Beat arcade mode with 0 deaths
Beat arcade mode without collecting a single crystal
Publish a map to Steam Workshop
Find an easter egg
Beat arcade mode
Kill the same opponent 3 times in a row
Beat the tutorial
Unlock arcade mode
Collect all stars on all levels