Here is the full list of all 18 Costume Quest 2 achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Found the Solar System
Tried every Creepy Treat Card in combat
Completed all quests in the game
Collected all Creepy Treat Cards
Mastered all combat techniques from Corvus
Used every costume special in combat
Found the Wolfman
Restored the timeline to save Halloween
Finished the game having fought every battle with Candy Corn in your party
Got the liberty kids to bust a move
Defeated Ophirion
Used the Pterodactyl Costume to flap 10 flappy things
Bashed 10 Monster Patrols
Upgraded 3 costumes
Collected over 4 thousand candies
Used the clown horn 1000 times
Defeated Kronoculus and his Kronys
Tried on 5 different costumes