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