Here is the full list of all 18 GORSD achievements.
Defeat this effigy.
Beat the game on Easy or better.
Beat the game on Normal difficulty.
Play a local PVP game with 4 players.
Play ten local PVP battles with 2 players or more.
Play fifty local PVP battles with 2 players or more.
Use the power vested in you by Cupiditas to illuminate the world.
Beat 100 stages in battle mode.
Beat 1000 stages in battle mode.
Complete the tutorial.
Transform into a tentacle.
Defeat 250 enemies in adventure mode.
Collect 100 bullets in adventure mode.
Collect 500 bullets in adventure mode.
Beat 500 stages in battle mode.