The Rotten King can be found in Agarica Palace
I used the Third Eye Tomb to defeat him.
He will dash towards you and throw magical spheres. He will also use a big circle of energy, energy arrows that split in 3. Basically it should be the same strategy as the last bosses I described about. Keep your distance, don't let him close the gap and keep hitting him.
Here is a video of how I defeated him: