Here is the full list of all 26 Gensokyo Odyssey achievements.
Somehow get one million dollars in Blackjack
Get 50k in Blackjack
Get a thousand dollars in Blackjack
Bring everything back to normal
Unite Gensokyo back with Earth
Get a Jackpot during Kakkoi's fight
Defeat Reiuji Utsuho
Defeat Hisoutensoku
Defeat Rin Satsuki
Defeat Byakuren Hijiri
Defeat Son Biten
Accept Kakkoi's bribe
Remember one tiny piece of information
Defeat Cirno
Get arrested by the FBI in the lamest way possible
Defeat Eirin Yagokoro
Become Eirin's Apprentice
Defeat Kakkoi and Utsuho
Defeat Remilia Scarlet
Take the Scarlet Devil Manion's hospitality for granted
Defeat Yuyuko Saigyouji
Solve the Bone Riddle
Defeat Kogasa Tatara
Defeat Marisa Kirisame
Complete the tutorial
Wake up for school