_npcscan ofc. And be sure to clear the cache regularly.
But also, if you check their possible locations on eg wowhead, they will all be up quite a lot, since noone is in those places anymore. So if you fly around their spots like 2x per day, im sure you have them all within the week.
The only one that is actually still hard is Doomsayer Jurin. Since his patrol route gets too close to Shattrath, so the Shatt guards kill him when he gets there. All the other mobs are generally up for a long time (hours?) before anyone kills them.
Pick up _NPCScan Overlay also. It shows the paths of the rares on your map and minimap. So much easier than trying to guess the path from comparing your map to Wowhead's.