Silhouette Mirage/YMMV

Game Breaker: Surosa. At Level 1 it's your basic pea-shooter, ineffective against most enemies but consuming very little energy with each shot. At max level, however, it becomes far and away the best weapon in the game with automatic rapid fire, very high damage output, and has no equal in draining Spirit from enemies to add to your own supply. It can even wipe out 90% of the bosses in the game in mere seconds. The best part? You can earn this weapon at max level in the second section of the very first stage of the game (just find the hidden shop at the top of the cave and grind out a mere 300 gold from the constantly-respawning Spectres nearby).
Mercy Mode: The easier difficulties only become available once you've completed all five story paths on the game. Better yet, once you enter the newly-unlocked Options menu, you learn that you were playing the second-highest difficulty the entire time.