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