I agree that Khem Val is not the best choice for downing Horak Mul. I never tried using Ashara or Andronikos for that fight because they are honestly undergeared. I always chose the upgrades for Khem and wanted to wait till I was 50 before gearing out my DPS companions. I figured DPS was the main component to the fight and I needed to burn him down fast. It became obvious that me healing a tank the whole fight was counter-productive and if I wanted him to go down fast I needed to do it myself.
I proceeded to grab Talos, put him in healing stance, and let lose with all my cooldowns. Went in 3 levels under Horak Mul and even surprised myself at how easily I beat him (although it was still dicey at times, but Talos came through with some clutch heals right after I shielded myself). Loved the fight, and I love those challenging 1 on 1 battles with elites.
Xalek was worth it, and I don't mind that he is a mute. I am hopeful that in future content he will begin to open up more.