Here is the full list of all 15 Oppaidius Summer Trouble achievements.
Started the game for the first time.
Discovered your spirit animal.
You didn’t miss them, did you…?
Went to the arcade with Serafina.
Won the swimming race.
Lost the swimming race.
Made Serafina’s breasts bounce through the power of your mind.
Waited for 2 minutes at the right moment…
Discovered the secret of the yellow dinosaur in Bermuda shorts.
Achieved a “Max Combo” hit count in the massage minigame!
Got ending “A”.
Reached the “Hot Secret Scene”!
Got every ending!
Set “boobage level” to 100.
Congratulations! You unlocked every art in the game!