True Anemone from Valdis Story: Abyssal City.
First, you have to deal enough damage to all three of its tentacles so that you can jump on them in order to reach the head. Then, when you're standing on the third tentacle, you have to jump and hit the head a bunch to try to break its shield. If you don't do it fast enough, the tentacles will regenerate and you'll get knocked off. Then, when you get about halfway, the other side of the stage turns into a crystal that heals Anemone, so you have to break that within the time limit too. (It heals Anemone for like 1/4 of its total health before you can even break it.) Once you break the shield around the head, you enter a short sequence where you basically play baseball with the orbs Anemone throw at you, sending them back to damage its actually health bar. (THIS IS THE ONLY WAY TO ACTUALLY DAMAGE IT'S HEALTH. AND HALF THE TIME THE STUPID HEALING CRYSTAL HEALS MORE THAN YOU CAN EVEN TAKE OUT BY DOING THIS.) Once you've dealt the miniscule amount of damage the game allows from hitting back all the orbs (Assuming you even hit them all back) The shield around the head, the tentacles, and the healing crystal all respawn and you have to do it again. And again. And again. And again. And again. Also, the whole time that you're trying to break the shield around the head, Anemone's shooting all sorts of attacks at you in numbers and patterns like those of a vertical scrolling shooter game. Something like this:
There's also a giant ice tornado covering the middle of the screen that freezes and damages you if you get caught in it. And when you're frozen, you can't really move, so it freezes you again. And again. And again. Until the miniscule knockback from it finally knocks you out of the tornado. And then there's the fact that if any of Anemone's projectiles hit you, they'll ALSO stun you for about 2 seconds, during which time you'll be pulled into the tornado and stunned and damaged again and again and again. Trust me when I say this is the most frustrating stupid and overpowered fight I've done in any video game.