Erubetie

Erubetie is one of the Four Heavenly Knights, ranked second. The Queen of the Slimes, Erubetie is embittered over human's pollution of water, drastically reducing the slime population in the process. She is calm and emotionless, and often keeps to herself, as she did not appear in Chapter 1 (having "shut herself" in Undine's Cave). She is also Undine's closest friend.

Manipulator of Water, she can use her body to create powerful whirlpools. Her most infamous attack is Melt Storm, which deals a whopping 1,500 damage and can easily defeat Luka in one hit (in both battles, if he does not have the proper guarding items).

Hateful of humans for their actions, she will kill them without mercy.

When she meets Luka, for the first time, she aims to kill him, but Undine stops her. Erubetie leaves, letting Undine deal with the human. In the Monster Lord's Castle, she fights him after Alma Elma and Tamamo, and unlike them, she won't show any mercy. During their battle, Luka tries to convince Erubetie that the humans mean no harm, but refuses to listen. When being backed down, she quickly divides in order to attack from all angles; unlike the Kistune and her Clone Jutsu, the Erubetie Triplets are connected to each other; when Luka attacks, another one sprouts in its place. Salamander chimes in and tells Luka that he has to defeat all of them simultaneously; her power gets boosted and he is able to defeat the Triplets with Vaporizing Rebellion Sword. In desperation, Erubetie clings onto Luka and transforms into a bomb, which can potentially wipe out the castle. Luka convinces her to stop as she's merely throwing both of their lives away, and by trampling on with the lives of other monsters, she'll be no better than humanity. Realizing her intentions are senseless, she backs down, seeing how pointless their fight is.

Strategy
In the first battle, immediately use Gnome and then Guard. Wait a turn, and Undine will interveine.

In the second battle, she hits unusually higher than Alma Elma or Tamamo. The use of Undine and Serene Demon Sword can help shave off a good amount of her HP before backing down to the Sylph + Gnome + Vaporizing Rebellion Sword combo. However, even with Sylph, there is still a high probability that Luka will not avoid it. Meditation is strongly suggested. She has three attacks that can easily screw Luka over: Melt Storm, which deals insane damage (800 with Gnome), Heaven's Prison which requires to attack to break free, however if she uses Neverending Frontier and Sylph isn't up, Luka will automatically lose, and Divide Destiny, which requires three turns of struggling otherwise Luka will lose. When her HP is shaved off enough, Erubetie will start dividing into Triplets. Perform a normal attack, and the defeated slime will be replaced. Use Salamander afterwards, whose power will be boosted. Then use Vaporizing Rebellion Sword to eliminate the Triplets. Erubetie will then grab onto Luka and put him into an unbreakable bind for three turns. After that, she will let go and the battle will be over.

Trivia

 * Despite the fact that she is a slime, she does not have any healing abilities.