Phagoycte

A single Phagoycte acts as one of the more unique boss fights of Final Fantasy Legend II, while shrunk down to microscopic size the heroes of the stories must collect magi from Ki's body while fending off shrunken minions of Ashura (who had been slain by this point in the story): however this single Phagoycte acted as a guardian of sorts of a Mana magi located inside Ki - the heroes fought the cell and managed to defeat it in order to gain the Mana magi, after which they exited Ki and became full-sized again.

Note
A Phagoycte is a type of white blood-cell found in the human body that protects the body via ingesting harmful bacteria, foreign particles and dead or dying cells.