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