My first entry for the EBF4 Foe Competition. Like I promised, I'm no good at this stuff (I STILL only have Paint) , but I figure the quality of the idea is what matters. So, let me go into detail about how this would basically work in the game.
Rainbow Slime (Actually, it's a Giant Rainbow Slime, but whatever) would either be a mini-boss or a boss. As you can see, they are composed of the different colors of the rainbow. In this form, it has a variety of different attacks, based on its colors, but if you hit it with a physical attack (e.g., Matt's standard attack) , you literally cut off one of its colors into a super-powered slime (Normal Slime, that is) . While red, yellow, green, and blue already exist, orange and purple do not. These could be Bomb-type and Wind-type, respectively. If you do not kill the slime within "X" amount of turns, it will merge with Rainbow Slime again. Once you slash off all its colors, it shows its "true" colors, solid black. It uses dark-based attacks now, but it can also summon normal slimes for it to merge with.
So, yeah, here's my first entry. may be more to come, but...maybe not.