What the achievement asks
Reach Exalted with the Uldum Accord. Five points, account-wide, both factions.
The Accord is Uldum's patch 8.3 faction, and its reputation comes almost entirely from the zone's assaults and the daily quests that come with them.
It is paced by the assault rotation, not by effort
Only one assault runs in Uldum at a time, and reputation arrives with its objectives and dailies. That makes this a matter of weeks of showing up rather than a grind you can concentrate.
Do the assault every time one is running, not only when the Black Empire assault is up. All three feed the Accord, and skipping the two ordinary ones doubles how long Exalted takes.
What pays best
The assault's own objectives are the bulk of it. Beyond those, the daily quests inside the zone and the emissary when it comes round are the rest.
Anyone working through Uldum Under Assault is doing exactly this work already, so the two finish together rather than competing.
Its counterpart
The Vale of Eternal Blossoms has its own faction, The Rajani, reached the same way through the Vale's assaults. Both are needed for the patch 8.3 category, and since the assaults alternate between the two zones, the sensible approach is to take whichever is running rather than favouring one.