beat him at 23. it's simply a matter of proper skill management
I went at him at 22, spamming everything when it came off cooldown, and got slaughtered each time
at 23, I took a look at my skills and planned out what I was going to do and when.
I sent Kira in, with Blade Ward active, and let her take the first hit. once Valis started casting Lightning Strike, I used Force Leap, which interrupted this and saved Kira some health. I then positioned myself between him and his ship. Open with Zealous Strike, Cauterize, and Blade Storm, followed by alternating between Strike and Slash as necessary. Once he used Energy Blast, my positioning allowed me to interrupt it almost immediately, and then repeat the above. Right after he uses Energy Blast his the best time I've found to use Master Strike, since it gives you the most time to make full use of it, instead of running the risk of having to cut it prematurely to interrupt something. After this, positioning wasn't as big a deal, as Force Leap was off its cooldown. Force Kick is now saved for Lightning Strike, while Force Leap is used on Energy Blast
tl;dr: keep Cauterize up, Zealous Strike as soon as it's off cooldown, interrupt Energy Blast and Lightning Strike, use Master Strike right after interrupting Energy Blast