A giant rainbow slime with a giant moustache.
For the King Sliamond boss, definitely use the Metal Killer sword. You don't even need legendary, as it only takes a few more hits after the proc to kill him. If he does flee though, force quit but choose to continue. If you follow your exact steps from before, he will flee at exactly the same point, but if you use an item before then, his actions will change. Use a boomerang or bug spray when you start the fight and he won't flee at the same time any more. Of course there's a chance he will flee earlier, but usually that's not the case.
Other tips that can help:
- Bug Spray is a good option to get a last hit before they have a chance to flee. Bonus, also useful to kill an enemy with "Hang in There!" before Flee (Pharaoh) or Pinch Crit (Goblin King).
- High accuracy weapons: Legendary Galactic Baton primarily, Legendary Fly Swatter early game
- High critical rate (Legendary Crystal Sword or Heck Queen set) to overcome their high DEF/dodge rate (technically an improvement but overall crit rate itemization is terrible)