So many of these guides are way out of date..

For the current build; Shadowbringers Patch 5.3 here's a rundown-

Leeches does not exist, it was redesigned as Esuna.
Selene/ Eos have been changed- setting them on Pet Hotbar to Guard now has either faerie use Embrace on the weakest party member. Also they use Whispering Dawn (AoE HoT Area of Effect Heal Over Time) on the entire team and cast Fey Illumination often as it buffs heal potency. No other skills we can set as Fey Blessing may replace Whispering Dawn but not for this quest as it's earned at 76.

So the- kite boss... failed. Focus healing- failed. I had 4 Augmented Hailstorm pieces on too (iLvl 240, Idyllshire in Dravania gear) as you must focus not on boss but his three Plague pets. So my strategy was simple.

Only cast Emergency Tactics > Adloquium at start, Bio the Boss and then it just becomes a case of watching. Use Succor to Barrier each in turn, then watch for plague pets. I got on boss's back so the Marines easily kept aggro and cast Art of War (160 unaspected damage to all under the AoE) until the minions were dead. That was it in the end :P .

All other guides to this I found were very much outdated with only the tip about getting at least four Idyllshire pieces being relevant at all.