Here is the full list of all 66 Crystal Story II achievements.
Complete the Secret Quest.
Defeat the boss of Giant Land.
Complete the bestiary.
Defeat the boss of Lao Prison.
Defeat the boss of Sunken Ruins.
Win all Mercenary Defense events.
Catch all the rare fishes.
Defeat the Card Master in a card battle.
Obtain the Number One Card.
Complete all Wanted X Quests
Defeat Calloc.
Defeat Mechaslime.
Defeat the boss of Doom Lair.
Use "Death's Gate" in battle.
Use "Trigger Happy" in battle.
Use "Cyclone Dance" in battle.
Defeat Chimera Prime.
Use "Dragon's Rage" in battle.
Defeat Seraph.
Defeat "The Big 3" in the Battle Arena.
Complete Chapter VI.
Defeat Foxer.
Complete all Wanted Quests
Defeat the "Snowy Chillers" (Hard Mode) in the Battle Arena.
Defeat the "Highlands Flyers" (Hard Mode) in the Battle Arena.
Defeat the "Swamp Stalkers" (Hard Mode) in the Battle Arena.
Defeat the "Earth, Wind and Fire" in the Battle Arena.
Win 5 Class GM Card battles.
Win 5 Class S Card battles.
Defeat the "Desert Crawlers" (Hard Mode) in the Battle Arena.
Use "Dragon's Breath" in battle.
Use "Magic Bullet" in battle.
Use "Lucky Strike" in battle.
Defeat the "Small Town Gang" (Hard Mode) in the Battle Arena.
Use "Hurricane" in battle.
Complete Chapter V.
Defeat the "Snowy Chillers" in the Battle Arena.
Defeat all match defense quests.
Get a Gold time on Track 3 of Snowboard Extreme!
Successfully hack all of the hackable chests.
Complete the Trickster tree.
Complete the Healer tree.
Win 5 Class A Card battles.
Defeat the "Highlands Flyers" in the Battle Arena.
Complete Chapter IV.
Complete the Thief tree.
Complete the Mage tree.
Use "Rend" in battle.
Use "Snipe" in battle.
Use "Cursed Dagger" in battle.
Defeat the "Swamp Stalkers" in the Battle Arena.
Complete Chapter III.
Use "Dragon's Roar" in battle.
Win 5 Class B Card battles.
Defeat the "Royalty" in the Battle Arena.
Complete the Guardian tree.
Complete the Slayer tree.
Defeat "The Dead" in the Battle Arena.
Defeat the "Desert Crawlers" in the Battle Arena.
Complete Chapter II.
Defeat "The Small 3" in the Battle Arena.
Complete Chapter I.
Win 5 Class C Card battles.
Defeat the "Small Town Gang" in the Battle Arena.
Win 5 Class D Card battles.
Win your first battle.