The first out of eight fake Evil Kings, the Sewer Evil King is a giant rat, who joins the circus after being defeated. He resides in the sewer system of Madril.
He also appears in the game's epilogue, being used for a circus trick by Epros .
Rosalyn will join Ari's party as soon as the fight begins. She deals a fair amount of damage, so it's a good idea to keep her on the offensive. However, her Agility is very high, so she will take her turn before Ari in most cases. If Ari is near-death, it's probably a good idea to have Rosalyn use a Nut on him.
It's a good idea to take out the Lurking Rats as early as possible. The best way to do this is to have Ari cast Overdrive on himself, and then use the Burst command. Keep in mind that this will cost a portion of Ari's HP, so have Rosalyn heal him afterwards. If Rosalyn's turn comes up before Ari can use Overdrive, set her to Wait.
Once only the boss remains, have Ari cast Overdrive on both himself and Rosalyn, and use Combine attacks on him. He should fall quickly.
The Sewer Evil King can kill Ari in a single hit if he is not properly equipped, or if he has a very low level. Don't be afraid to go out and grind a bit if he's giving you trouble.