Here is the full list of all 18 Panty Party achievements.
Clear all arcade stages.
Finish arcade route 6.
Finish arcade route 5.
Finish arcade route 3.
Clear a single arcade route with a highscore.
Finish arcade route 4.
Finish arcade route 2.
Clear all arcade stages as fast as possible.
Finish arcade route 1.
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.