Darach defeated Vindican, who's sub-Maul tier, while fending off Return Malgus, likely Maul-tier, at the very least slightly superior to Vindican. Luke will be absolutely slaughtered, it is completely beyond him to parry Kao's single strike.
Force only Darach could Force Crush Luke's skull to pieces, but it's obviously not within his arsenal as a Jedi, and on neutral ground Darach can't do anything else with the Force to hurt Luke; we only saw him use regular Telekinesis (albeit he is as superior as I noted), but it won't allow him to kill Luke. Dunno what to tell.
All out - just like sabers.
Luke was stated by Vader to be his strongest opponent yet, I'm inclined to give Luke the win here, but idk how reliable that really is given he was in emotional turmoil over whether to kill his son or not. I'd give sabers to Darach, seeing as he's the Jedi Battlemaster and took on Malgus and Vindican briefly, Force would probably go to Darach, too. I don't remember Luke doing anything on par with Darach (while seemingly depleted) hurling starship parts. All-out again Darach. I'd like to see some feats for Luke that could change my mind, if someone could provide
Luke imo. In Legends he resisted a TK blast from Vader, gave him a great fight in ESB to the point where he even pierced Vader's defenses and in general just has the benefit of just being younger and having a greater connection to the force which would allow him to outlast Kao if nothing else. I don't think Kao could overwhelm Luke like Vader did. Also Luke was able to instantly adapt to Vader's form and fighting style. Luke's pretty hackzed out when you think about it. Still think it's a great fight and it could be argued that Kao's superior skill and versatility could off balance Luke and give him the win. Going with Luke though.