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