Nickelodeon All-Star Brawl 2

Nice trick but I’m really struggling on the jellyfish boss ? Any tips Seems really difficult did you do it this way ?

It's not my solution, but helped me.

Note: This achievement does NOT require you to beat the entire Campaign on the highest difficulty. That would be insanely difficult. It requires you to finish a run in Campaign mode on the highest difficulty. Savior of the World (100G) explains to layout of Campaign mode and how runs work. If you read that achievement solution, you'll understand why the easiest way to get this achievement is to do it at the start of a new Campaign save file, on the first run of the Campaign. This is because the first run is the shortest and easiest. The first run consists of around 4 or 5 fights and a boss fight against King Jellyfish. To put it in perspective, regular runs consist of at around 20 fights and three boss fights, so it would be incredibly difficult to do that on the highest difficulty.

So, start up a new save file in Campaign mode on the highest difficulty. This part is set: you'll be playing as Spongebob, and will have the same fights: an arena fight, a fight against Patrick, another Arena fight, and a fight against someone else of your choosing (depending on the path you pick), then ending with a fight against King Jellyfish. You need to do all of that on the hardest difficulty without dying once. Any damage you take in one fight carries over to the next too. As a result, your priority needs to be getting Blimps and spending them at shops to heal whatever damage you took. Play all fights safely: stay in the center and do charge attacks while waiting for enemies to approach you. The Arena fights are pretty simple, against small jellyfish that don't even attack you often on the highest difficulty. The fight against Patrick can be dangerous. Don't be lulled into a false sense of security. His attacks will do very high damage and knockback to you, so be very careful you don't let him get close. Same deal for the other arena fights, and for the second opponent fight.

Once you get to the King Jellyfish fight, hopefully with relatively low damage (under 70% or so) so you can take a couple hits without being knocked off screen, you've reached a checkpoint of sorts. With all of the other fights, if you die it's over; you need to delete your save file and start again. Once you get to the King Jellyfish fight, if you lose it doesn't save your game immediately. If you die, King Jellyfish will start with some dialogue. During this dialogue, force close the game app. When you reload the game, you'll be back at the King Jellyfish fight, giving you unlimited attempts to beat it.

For the fight itself, it does four attacks:

Ground smash, which is highly telegraphed. Move out of the way, jump over the shockwave, and attack
Laser, which happens fast without much time to reach. Jump over it
Energy ball, which is the worst one, because if it hits you, you're pretty screwed on the highest difficulty. That thing will send you flying. Avoid at all costs
Charge across the platform. This is the rarest attack. Jump over it with double jump and then an 1LSUp.gif+button-x.png recovery
The fight seems to always start with a ground pound attack, then it moves to the left and does a laser then an energy ball. I would jump into the ground pound attack and attack it, then go right on top of it and charge attack it twice. Then, to avoid its laser and energy ball attacks, either double jump off the edge of the platform and then back, or hang down off the edge. After each attack, get in two charge attacks. After the energy ball attack, you should be able to keep charge attacks going to get the boss to half health, at which point it'll charge up for the second half. For the second half, same strategy: stay on top of it, and whenever it is about to attack, double jump off the platform and back so you don't get hit. Between each of its attacks, get in two charge attacks.
With any luck, you'll win this fight within a few attempts of learning the attack patterns. It took me probably 10-15 attempts, so be patient and don't be too aggressive. I went into the fight with 56% damage, so I was able to take two or three hits and still recover, as a point of reference. If you're going in over 80% damage, you might as well restart the run, because that essentially means you need to beat the entire boss fight without getting hit, which is pretty rough.

Once you beat King Jellyfish, you'll unlock this achievement, which is probably the hardest in the game.