Been doing duo runs for Al'ar, Azure and Onyxian drake weekly with a warrior a sp/disc with 47 kills on kael, 48 kills on maly, 40 kills on ony. Al'ar dropped on 38th run (lost the roll), Azure dropped on 41st from 10man maly (mate passed it after winning Al'ar previously).
I realize you can solo kael as an SP now, but it goes so much faster and safer in duo anyway, so here goes:
Have your warrior go Arms or Fury, priest goes disc/holy.
Phase1: just nuke the advisers one by one. Try to pull and kill all of them next to the left statue, except for Solarian.
Phase2: focus down Cosmic Infuser and then AoE all the other weapons. Priest equips staff, warrior picks up shield, but doesn't equip it yet.
Phase3: Warrior AoEs the three advisors, while priest tanks and maybe even kills Solarian with Smite/Holy Fire. The warrior may need a few heals occasionally.
Phase4: Priest LoS behind the left statue, while warrior pummels away at Kael on the right side of the left statue. Experiment with LoS to see from where you can heal the warrior/be in LoS for Kael after his first "Obey me!", which was his MC. Throw heals/shields on Warrior between MC casts.
Phase4-5 transition RP: Priest has to stay in LoS, as the boss will still occasionally cast his MC.
Phase5: Nuke boss, stay out of the dark blobs, gg.