The oly reason I would say Kun and Qel-Droma would have a chance is because Nadd nearly destroyed Sadow's spirit. So Kun might be able to persaude Sadow to attack Nad which would mean it could end up being Exar and Ulic vs. Sadow (as they would promptly betray him.) That wouls probably be a fairly even fight.
I'd like to note that Ullic is not weak. His powers are primarily combat oriented however, while the other three are all SIth Sorcerors of great ability. Had Nadd and Sadow been alive he would be far more effective. As the oter two are ghosts this is no longer the case.
Kun would probably not be able to defeat Sadow alone when he firsts comes to Yavin. Sadow has studied Sith magic for years. At the end of Kun's reign it is obvious that Exar has yet to fully grasp the powers he wields. (He says this straight out at the end of Sith Lords. He also unintentionally trapped himself in his temple (and if you say this is not so then explain the desperate 'ULIC!' on the last page...) He may not have even meant to completely destroy the planet!)
This is why the only way Kun wins is if he tricks Sadow into helping with Nadd and Ulic is still standing when the engage the second Sith.