Build up throughout the game - Will unlock on Level 21 if done correctly
Missable & glitched achievement
In order to get this achievement at the end of the game, be sure to use Futo at least once on each level to kill an enemy (except boss fights). If you happen to forget to use Futo on a level, take a note of which level. Certainly DO NOT replay the level.
Once you've beaten the game, you can replay previous levels.
NOTE: If you're not sure which level(s) you could have missed, then simply replay each level over again from the beginning, kill at least one enemy as Futo, and then save your game at a checkpoint, quit out and then move on to the next level.