After completing the game, load the cleared game file. Head to Mt. Zugzwang and walk south into the poison swamp to go inside Estark's Lair. Defeating Estark will unlock the Stark Raving T'n'T board game. If you win the board game, you also unlock Starkers as a character. Afterwards, go to the Pothold to find Rebjorn as an available character.
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.
When talking to Saber, if he says "Grrr....Prrr", it means the current area is safe. If he says "Grrr....", it means that the current area is not safe. Be sure to use Saber when entering new rooms in a dungeon.
When you get to Bishop Ladja at Talon Tower, makes sure you're at least around level 34. Keep your son at the back of your party so he doesn't receive damage often. Have Saber at "Show No Mercy" and use Oomph on him when he increases his attack power. be sure to use Kazing to revive your party if they die. Keep attacking Ladja while healing and reviving, as well as using Oomph on Saber when he increases his attack power, to defeat Ladja. Note: If you're at level 37, you can also try setting your party (Saber, your son, and your daughter) to "Show No Mercy".
Metal King Slime's locations: Mt Zugzwang and Estark's Labyrinth. Best spot is a floor that you would fall through and basement at Mt Zugzwang, a swamped floor (just a floor before Estark) in Estark's Labyrinth.