If Luke can pin Caedus to his chair without even gesturing and then ragdoll him through his starfighter in the middle of a dogfight, and Caedus > Vader, Luke could ragdoll + electric judgment Vader quickly. Then it's Caedus vs. Krayt (Caedus wins in a long fight) and Luke vs. Malgus and Kun (Malgus is fodder).
He has the speed advantage which means he gets the first strike, which would definitely be enough to dispose of Malgus and probably be enough to kill any one of the trio. Then it's just Luke vs. two. There's plenty of precedence for this: Dooku vs. Anakin and Obi Wan, Sidious vs. Yoda and Anakin in that vision, etc. The weakling doesn't always contribute much.
Also I don't have Malgus that far from Satele, who got ragdolled by Revan.
The other possibility is that Caedus engages and quickly destroys Malgus while Luke takes on the other three; given how long Luke survives against Abeloth multiple times, he could at least survive long enough for Malgus to get ragdolled.
He just needs to one-shot Malgus, and then he can take Kun and Vader together, seeing as how it's implied that TFU Sidious could defeat Vader and Starkiller together.
Also, seeing as how Luke has ragdolled Caedus twice, it's not that difficult to believe that he could one-shot Vader, particularly given his weakness to lightning.