Robo-SpongeBob MusclePants is an unused boss in Battle for Bikini Bottom. The phase crashes due to broken collision in his Anchor Arms, but replacing these broken files with blank files will fix the problem. There is no other way to damage him other than popping his Anchor Arms.
The boss originally had footage of this phase in very early trailers of the game in its beta, where it showed SpongeBob running away from it. The boss there was shown to have green eyes and its sclera being much less transparent, but eventually, the internal view from its first phase was used for the model in the hack.
This boss also cannot damage the player character, as all of its attacks have no hitboxes.
He merely looks like a giant version of SpongeBob with Anchor Arms and a blue swimsuit. However, unlike its previous forms in the final build of the game, the SpongeBot here is considerably smaller, and also has a pair of legs with SpongeBob's signature socks-and-shoes combo. The generators from its first phase used in the final build are also absent.
Smash: SpongeBot's main form of attack. The robot brings its Anchor Arms down in front diagonally apart from each other, which causes him to be suspended in midair while having its eyes in a wonky state. Much like the used-version of its first phase with his horizontal chop, it is left vulnerable for a while during this. Targeting its Anchor Arms with abilities like Patrick's Throw (on the debris of the arena) will cause the respective arm to pop and deflate. Eventually, the SpongeBot will recover with another smash to vault itself back into its default position, arms hit or not.
This attack seemingly has an unknown hitbox, since it mostly smashes on both sides of the player's position. This attack will still be used even if both Anchor Arms are deflated.
Stomp: If the SpongeBot is approached at close quarters, he will eventually hop and repeatedly stomp the ground. After a few of the said stomps, continuous shockwaves (with the color akin to SpongeBob's bubbles) will emerge from both feet stomping at the same time each.
Just like its other attack, it has no hitbox regardless, and the waves are somewhat too tall for any of the playable characters to jump over even with a double jump. The shockwaves emerge 5 times before the appear again after a few more stomps. The boss will not stop stomping until the player moves out of its range after the shockwaves emerge.
This battle is of course, unwinnable, as the SpongeBot cannot deal any damage nor can it be defeated (it also lacks any sound effects); it must be somewhat damaged through popping its Anchor Arms with Patrick's throw ability but can be "damaged" no further than that. SpongeBob can also aim his Cruise Bubble at its right eye from a distance where it shows a targetable area, but nothing happens when the attack hits its mark.
Also during the said unused phase, if Sandy is approached with a type of terrain-traveling mod and if hit by an attack, she'll perform a pain animation different than the one used in the final game.
The battle always starts with Patrick being selected first, as his throw ability is most-likely needed to pop/deflate the robot's Anchor Arms.
The SpongeBot has two different sets of animations for both arms deflated or not deflated, and it also has an extra idle animation and two taunting animations.
Since the unused fight is playable with Patrick it is possible to have multiple phases with each other playable characters SpongeBob and Sandy with their own attacks during the fight just like Robo-Sandy and Robo-Patrick boss fights.