Mages in Maplestory: Use Magic Guard and drink your potions as necessary, keep an eye on MP if runs you will get hit much harder than you expected. Keep attacking him with the Magic Claws.
Assassins: Dexless assassins with Ilbis/Steely's will have a wonderful time with the Slime King; you will quickly bring his massive HP down with your deadly and accurate attacks. But do not be too confident, you still do not have much HP to take his attacks with. Remember your jump attacks to avoid his area strike. By the way, we provide Maplestory powerleveling for you.
Archers: You have a long range, a nice cushion of hp(MapleStory Mesos) and a high accuracy. You will just sit and pump out the Double Shots. However, you will get hit by the area strikes, you can not jump attack very well at all. Remember your potions and you will be fine.