First, power up your party members. Then, usee Kabuff and Oomph. Unleash your strongest magic (Kazap for your song, Kaboom, Kacrackle, etc. for your daughter). Korol likes to use Bounce so be careful. Do not use Show No Mercy. Make sure you're around level 40 or higher. Also, try to have the Zenithian equipment for your son, the Princess's Robe for your daughter, and the Hero's Cape for yourself. It is essential for you to have these items to defeat Korol. As for weapons, if you have the time, get three Metal Slime Swords from a casino along with two Gringham Whips. Equip Metal Slime Swords for you, your son, and Goodian (Slime Knight). Also, be at full stats (full HP and MP). You should reach Korol with little to no battles at all if you memorized the way. Run away from battles to keep your MP and HP at maximum.