Jelly King, and the hopeful future!
I like the puzzles preceding him, and him in general but, he. and his previous levels are still very easy.
I propose we spice it up a little bit by adding a random element to him each time he appears.
IE: Elemental based stages. Shock, Fire, Freeze, Poison, Normal.
Can you imagine the challenge of the areas if the respective slime families are present in the current stages match a certain element each time? I can, and would love to see it happen.
That being said, something can be done for the Jelly King too. Have him, in his majestic glory, randomly during the fight switch his own element and the surrounding jellies element throughout the fight. That would certainly spice up the fight quite a bit. Oilers, Quicksilvers, Ice Cubes all in abundance all around him while he tramples knights left and right. Give him abilities based on his element and you're set for a polished boss fight.
Challenging, but possible. ^.^
Also if you want to make him into a phased boss to make it take longer than the current zerg it to death strategy, make it start as normal element and change its element every 20% of its hp to one of the remaining elements that haven't been chosen yet.
Jelly King is located in Tier 2, and I have done many jelly kings run and easily Solo'ed him, but I use all 5* Weapons I think if you look at the requirements of Tier 2, and compare it to your gear its pretty easy with 5* gear, but using lower * gear its not a big challenge, but still harder. I personally, think just adding maybe a few gun puppies in the last stage may make it more fun or Jelly Trojans lol...but thats about it since its Tier 2 I would not like to see it become real hard for the players who are not ready for Tier 3 (Since Tier 3 is not a place most players can solo without spending a bunch of energy)