Who Was The Only Force User That Boba Fett Feared?

The renegade clone of Galen Marek, codenamed Starkiller. In Legends, Boba Fett took a contract from Darth Vader to hunt the clone down, and what he found on the way convinced him he was outmatched. Vader pointed him at the clone's lover Juno Eclipse instead, and once he had her, Fett demanded his fee on the spot because he expected to die if he stayed. (Star Wars: The Force Unleashed II; Star Wars: The Force Unleashed II (graphic novel))

Fett did not usually rate Force users#

Boba Fett spent his whole career around people who could move objects with their minds, and he made a point of not being impressed. He had fought Jedi before, and he treated them as a problem with a solution rather than a wall he could not climb. That attitude held up. He worked contracts against Jedi during the Clone Wars era and he worked for the Sith afterwards, and neither side left him rattled. (Star Wars: The Force Unleashed II (novelization))

Which makes the Starkiller job stand out. This is the one time the record shows Fett hesitating. (Star Wars: The Force Unleashed II (graphic novel))

The contract from Darth Vader#

Fett was living in relative poverty on Nar Shaddaa alongside a fellow bounty hunter named Xasha when Darth Vader got in touch. The Sith Lord offered a dangerous job and a substantial payout, and both hunters were short enough on credits to say yes. They took Slave I to Cato Neimoidia to begin the hunt, and somewhere on the way there Fett told Xasha who the target was: an extremely powerful Force wielder codenamed Starkiller. (Star Wars: The Force Unleashed II (graphic novel))

The two of them started at Tarko-se, or what was left of it. Starkiller had levelled the entire city. Picking through the wreckage, they found the ruins of the city's arena and, inside it, an enormous creature that looked dead. (Star Wars: The Force Unleashed II (graphic novel))

What the Gorog told him#

The creature was a Gorog, one of the most dangerous beasts in the galaxy, and Starkiller had very nearly finished it off. The arrival of the two hunters woke it up. It attacked in a frenzy, and Fett, caught off guard by how much fight was left in something that badly damaged, killed it by firing himself straight through its chest with his jetpack and taking out its backup heart. (Star Wars: The Force Unleashed II (graphic novel))

That was the moment the job changed shape. It was not only that a Gorog had been left in that state by one man. It was that the same man had done it and then walked away from a city he had flattened. Xasha had seen enough and quit the hunt there, and she tried to talk Fett into leaving with her. He refused, partly out of pride and mostly because he needed the money. (Star Wars: The Force Unleashed II (graphic novel))

Fett then flew to his homeworld of Kamino to meet Vader at the Timira City Cloning Facility. After watching a holo-recording of the target escaping, Vader told him the rest: this was not Galen Marek. Marek was dead. The thing Fett was hunting was a renegade clone, one of a run of failed experiments Vader had grown from his late apprentice. The clone had inherited Marek's power and his memories, and none of his stability. (Star Wars: The Force Unleashed II (graphic novel))

Going after Juno Eclipse instead#

Vader knew Fett could never take the clone head-on, so he pointed him at what the clone cared about instead. The one thing the copies of Marek could never shake was his feelings for Juno Eclipse, so that was where Fett went. (Star Wars: The Force Unleashed II (graphic novel))

He ambushed the Rebel ship Salvation and left with Eclipse as his prisoner. Eclipse tried to talk him round on the way out, and Fett's answer says everything about how he was treating the contract. "It's about repeat business," he told her, "and your Alliance most likely won't exist long enough to pay my first fee." She called him overconfident, like the Emperor. He said he had reason to be, because the Emperor's credit was good. (Star Wars: The Force Unleashed II; Star Wars: The Force Unleashed II (novelization))

Back on Kamino he tried to hedge. Fett demanded his reward right away rather than waiting for the clone to arrive, on the reasoning that he would probably not survive the battle. Vader refused and made him stay. For a man who had spent decades betting on his own survival, asking for the money first is the closest thing to an admission of fear in his whole file. (Star Wars: The Force Unleashed II (graphic novel))

Fett never did face the clone head-on. When the Rebels stormed Kamino, he spent the battle trying to line up a sniper shot from a distance while the clone duelled Vader. (Star Wars: The Force Unleashed II (graphic novel))

Where this sits in continuity#

All of it is Legends. The Starkiller storyline comes out of The Force Unleashed and its sequel, which Disney moved into the Legends banner in 2014, so none of it is part of current canon. The Force Unleashed material is also where the more extreme end of Starkiller's abilities lives, and it is the reason the clone reads as a genuine threat rather than another mark. (Star Wars: The Force Unleashed; Star Wars: The Force Unleashed II)

Fett's own later canon appearances give you the contrast. He goes toe to toe with people carrying lightsabers and treats it as a Tuesday. Set that against the Legends version circling a dead Gorog on Cato Neimoidia and quietly deciding to take the money in advance, and the difference is obvious. (Star Wars 6; Star Wars: The Force Unleashed II (graphic novel))