What the achievement asks
Kill fifteen of Azshara's champions in Nazjatar. Ten points, account-wide, both factions.
None of the fifteen can be hunted. Each one exists only while its own world quest is active, and there is no other way to make it appear.
The fifteen, and their quests
Why the coordinate lists do not help
Search for one of these champions and you will find a location. Go there and, most days, nothing is standing on it.
They do not spawn unless their world quest is up. The location is real but conditional, which is why people spend an afternoon flying between fifteen points and come back with nothing. Two of the fifteen are typically active on any given day.
Check the Nazjatar world quest list first, then fly to whatever is on it. Never the other way round.
Roughly a fortnight of checking
With two up per day and no control over which, fifteen champions takes a couple of weeks of looking in. There is no way to force a particular one and no reason to hurry: the achievement is patience rather than difficulty, and at current levels each champion falls in seconds.
The habit that finishes it is the same one that finishes the rest of Nazjatar. Open the map, see what the zone is offering, and take what you still need. Puzzle Performer and Subaquatic Support are gated by exactly the same daily list.