Here is the full list of all 11 TouHou Pants Contest achievements.
Collected the first pants
Collected the ten pants
Collected the One hundred pants
Failure one times
Failure ten times
Defeat kaguya for the first time
Defeat yuyuko for the first time
Defeat furan for the first time
Complete the battle at 0 frames
Complete all of the main event
All CG Successfully collected