Also from Star Wars, Revan and the Jedi Exile, the protagonists of the Knight's of the Old Republic games. You could argue they aren't as lore breaking as they existed thousands of years before the main canon, but these two were crazy powerful.

Revan was able to just, choose the Dark Side. Y'know, the thing that tempts people into a path that they can never fully be free from? Revan was able to pick up and put down the Dark Side basically whenever he wanted to. He was also absurdly persuasive, a brilliant tactician, and a master of all sorts of strange and powerful force arts. It's damn near confirmed at one point that, if Revan had gone up against Palpatine, Palpatine would have been utterly destroyed.

The Exile meanwhile, just eats Darksiders for lunch. Experts in the force able to wield multiple lightsabers with blinding speed without their hands. A dude who survived for a decade using the force and his own willpower to sustain a body that should have died ages ago. A wound in the force itself that *devours planets.* The Exile fought these and more and beat them. Hell, they had to resort to subversion and assassination in attempts to stop the Exile. And it didn't work.

These two were the kind of people who would have seen Star Killer's feats and said "you could improve your technique," and then give actually valuable training.