Sera Leader
The ultimate Reyvateil-type enemy, and also, the rarest enemy in the game, who only appears accompanied by the other three Sera enemies: the Sera Beast, the Sera Bird and the Sera Heart. Because of this, you will have to deal not only with one, but with four dangerous enemies at once. Her Girl Explosion attack causes Fire-elemental damage to all the party aside of causing Curse, which guarantees that all enemies will get critical hits. She also has a Lv. 1 Stored Attack called Li'l tonelico, which causes strong damage aside of ignoring defense, so make sure to Guard your Reyvateil properly.
As an extra note, all the Sera enemies are very rare and hard to find, yet if you want to synthesize all the items in the game, you'll have to encounter and defeat at least one of them, since they are the only way to obtain the Star Tiara, which is needed to synthesize Shurelia's ultimate armor, the Grand Crown.