What the achievement asks
Kill three bosses in Uldir: Vectis, Fetid Devourer and Zul, Reborn. Ten points, account-wide, both factions.
The three are not next to each other, and the last of them is the reason this achievement is bigger than it looks.
In practice, it means clearing six bosses
Uldir runs in this order: Taloc, MOTHER, Fetid Devourer, Zek'voz, Vectis, Zul, Mythrax, G'huun.
Taloc and MOTHER gate everything. After MOTHER the raid opens and Fetid Devourer, Zek'voz and Vectis can be killed in any order, so two of your three targets are free at that point.
Zul is the catch. He does not become available until all five bosses before him are down, Zek'voz included. So even though Zek'voz belongs to a different achievement, you cannot skip him here.
Counting it honestly: Crimson Descent costs you six kills, not three.
Which is why you should do it in one trip
Uldir splits into three achievements and the raid only counts towards the meta when all three are done:
- Halls of Containment: Taloc, MOTHER, Zek'voz
- Crimson Descent: Vectis, Fetid Devourer, Zul
- Heart of Corruption: Mythrax, G'huun
Between them they cover all eight bosses. Since Zul already forces you through the first five, clearing straight to G'huun in a single lockout finishes all three at once, and splitting them across weeks buys you nothing.
Crimson Descent names no difficulty, so any difficulty earns it. The parent achievement Uldir is the one the meta counts, and it requires Normal difficulty or higher. Run the raid on Normal and both are covered.