This is Dark Nest Luke.
Luke by the last book when he shits on Lomi Plo would be interesting, though Palpatine would still win.
Earlier in the series, definitely Palpatine.
I don't think Luke reaches that DE Sidious/Valkorion tier and beyond until FOTJ when he's tussling with Abeloth every other day tbh.
I haven't read the trilogy in a while, but I think it's just the mentality thing, which seems to make all the difference in Star Wars. When he has to go back and watch those recordings of Anakin and Padme in order to face Lomi Plo in the last book, though maybe that was just to deal with Lomi in particular due to her unique abilities. Can't really remember, I just know he roflstomps her super quick in the end.
I've found a quote stating that it was indeed Leia who allowed Luke to beat Sheev, even stating the twins combined defeated him. Which basically confirms Luke isn't doing so by himself.
Pregnant again, Leia faced another major crisis: Her brother seemed to have become corrupted by the dark side of the Force, thanks to the teachings of the cloned Emperor Palpatine. He tried to corrupt her, too, but she escaped with Palpatine's Jedi Holocron. Later she rejoined Luke, and together they overcame the clone's dark side power.
- The Complete Star Wars Encyclopedia
Skywalker was too enmeshed in darkness to successfully rebel against his master. Although he sabotaged some of Palpatine's military ventures-namely those involving immense war factories called World Devastators-he still could not draw himself from the pall of the dark side. It was his sister, Leia Organa Solo, who gave him the extra strength he needed. With her presence, the two Skywalker twins were able to temporarily repulse Palpatine.
- The Complete Star Wars Encyclopedia