What the achievement asks
Kill four bosses in Ny'alotha, the Waking City: Dark Inquisitor Xanesh, The Hivemind, Vexiona and Ra-den the Despoiled. Ten points, account-wide, both factions.
It is the largest of the four Ny'alotha wing achievements, and the only one that asks for four bosses rather than two or three.
The four are not consecutive
If you line Ny'alotha up in raid order, these four are the fourth, fifth, ninth and tenth encounters. The three in between belong to Gift of Flesh.
That is not an accident of numbering: after the opening three bosses the raid divides into two halls that run in parallel. Halls of Devotion is one of them, and the bosses you want here sit on either side of the other wing.
The practical consequence is that you are not going to finish this one without walking through Gift of Flesh territory as well, so plan on doing both in the same run rather than making two trips.
How the raid divides up
- Vision of Destiny: Wrathion, Maut, The Prophet Skitra
- Halls of Devotion: Dark Inquisitor Xanesh, The Hivemind, Vexiona, Ra-den the Despoiled
- Gift of Flesh: Shad'har the Insatiable, Drest'agath, Il'gynoth
- The Waking Dream: Carapace of N'Zoth, N'Zoth the Corruptor
Halls of Devotion names no difficulty, so Raid Finder earns it. The parent achievement Ny'alotha, the Waking City requires Normal difficulty or higher, and that is the one the meta counts.