Here is the full list of all 26 Gensokyo Odyssey achievements.
Defeat Son Biten
Remember one tiny piece of information
Defeat Cirno
Become Eirin's Apprentice
Accept Kakkoi's bribe
Get arrested by the FBI in the lamest way possible
Defeat Eirin Yagokoro
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
Defeat Byakuren Hijiri
Defeat Hisoutensoku
Defeat Reiuji Utsuho
Bring everything back to normal
Unite Gensokyo back with Earth
Defeat Rin Satsuki
Get a thousand dollars in Blackjack
Get 50k in Blackjack
Somehow get one million dollars in Blackjack
Get a Jackpot during Kakkoi's fight