Do you play on PC or console?
If you're on PC, maybe there's a method to make this easier with either mods or cheats, but I'd have to explore this further. Maybe you could use Windows speech recognition macros to bind a side dodge to voice commands, to more easily dodge Kai Leng's insta-kill.
If you're on a console, I can only offer general advice. If you are able to dodge his initial attack and keep your distance, you may be able to beat him with squad commands alone. So pause a lot and use squadmates to lower his shields.
I'm afraid I can't be more helpful.
