What the achievement asks
Kill three bosses in Uldir, the first raid of Battle for Azeroth: Taloc, MOTHER and Zek'voz, Herald of N'zoth.
Worth ten points, account-wide, and open to both factions. There is nothing to unlock first and no order to discover: Uldir gates itself, and the three bosses you need are on the shortest path through it.
You do not have to clear the wing
Uldir is only partly linear, and that works in your favour here.
Taloc must die first. MOTHER second. After that the raid opens up: Fetid Devourer, Zek'voz and Vectis can be killed in any order, and nothing forces you to kill all three.
So the whole achievement is three pulls: Taloc, then MOTHER, then straight to Zek'voz. Fetid Devourer and Vectis can be left standing.
Run it on Normal difficulty or higher. Halls of Containment names no difficulty of its own, but the achievement it feeds into, Uldir, says plainly: defeat the bosses of Uldir on Normal difficulty or higher. That parent is the one the meta counts, and it was added in patch 11.0.7 for exactly that purpose.
The two bosses you skipped will come back
Worth knowing before you leave, because it changes how you plan the trip.
Uldir splits into three achievements, and you need all three for the raid to count towards the meta:
- Halls of Containment: Taloc, MOTHER, Zek'voz
- Crimson Descent: Vectis, Fetid Devourer, Zul
- Heart of Corruption: Mythrax, G'huun
Zul does not unlock until all five of the earlier bosses are down, which means Fetid Devourer and Vectis are only optional for this one achievement, not for the raid as a whole. If you intend to finish Uldir in one visit, killing them on the way to Zek'voz costs you nothing and saves a second lockout.
After Zul come Mythrax and G'huun, and the raid is done.