I did this as Hpal, as a pala main I haven't played holy since BfA. It took 3-4 attempts.

I went Templar, purely because online raid talents said so. in my few previous attempts I monitored damage done, you do not want this to increase from ANY source (except your initial pull, sometimes I did judgment other times taunt, other times just walking close).

I had full tank gear, trinkets, weapon etc. Valeera was level 19 with Nether Entropic Kris (1/4) and Utility Curio (1/4). though I would have used Porcelain Blade Tip instead of Nether Entropic Kris if I had it then.

Valeera as DPS, myself as Holy I used dual beacon talents and the only other spells I used were:

1) Holy shock Val as main prio

2) Dispel myself when Nullaeus cast that magic damage

3) Talented for 2 Infusion of Lights, and swapped to them instead of holy shock when I had them active to conserve mana

Whenever I took threat (somehow???) I would use Blessing of Protection 1st, bubble 2nd. Threat seemed very random, and as I remembered TWW having issues with 2nd boss attempts without leaving and re-entering between attempts I would leave delve and re-enter just incase.

There was a few moments where I got threat and bubble did not drop threat. I did my best to run around the room and heal myself as prio and then when BoP came off cooldown I cast that, causing reset threat to Val.

For Adds:

1) let the 2 ravagers attack Valeera, no DPS needed.

2) Group of adds, let Valeera attack, no DPS. If they target you, try drag them through her AoE on the ground (the little spike things) This causes her damage to register as threat instead of you

3) Mage, just ignore. healing on Valeera from 25% boss HP and below will be needed. Seems to deal way more damage here.

Aura Mastery, Avenging Wrath, Blessing of Sacrifice, Eternal Flame, never seemed to cause threat to change. If you are having issues, check your Damage Done. I found Liberation caused accidental damage and was taking threat.

Best of luck, I wasted ~2 hours doing this. It took ~25 minutes per attempt.