Here is the full list of all 18 Panty Party achievements.
Finish the story mode.
Collect all Panzi-sama panties.
Collect all panties.
Collect all Taoism panties.
Collect all Mystical panties.
Collect all unpopular panties.
Collect all striped panties.
Collect all elegant panties.
Collect all bear panties.
Finish arcade route 5.
Clear all arcade stages.
Clear all arcade stages as fast as possible.
Clear a single arcade route with a highscore.
Finish arcade route 1.
Finish arcade route 2.
Finish arcade route 3.
Finish arcade route 4.
Finish arcade route 6.