What the achievement asks
Kill the last two bosses of Uldir: Mythrax the Unraveler and G'huun. Ten points, account-wide, both factions.
There is no shortcut to these two
Mythrax is the seventh boss and G'huun the eighth, and Uldir gates its ending strictly. Everything before them has to die first: Taloc, MOTHER, Fetid Devourer, Zek'voz, Vectis and Zul.
So this achievement is not two kills, it is a full clear. There is no side entrance, no skip, and nothing to plan around. Walk in at the start and work through to the end.
The good news is that a full clear also finishes the other two Uldir achievements on the way, so one trip closes the raid completely.
The three that make up the raid
- Halls of Containment: Taloc, MOTHER, Zek'voz
- Crimson Descent: Vectis, Fetid Devourer, Zul
- Heart of Corruption: Mythrax, G'huun
Heart of Corruption names no difficulty, so any difficulty earns it. The parent achievement Uldir, the one that feeds the meta, asks for Normal difficulty or higher. Run the whole thing on Normal and you are done in one lockout.