Terror Wraiths only spawn in one group of 2^^, 3^, or 4 regulars at a time. This spawn is on a 10-20 minute timer. You can however make the progress much smoother if you manage to get 4 of them to spawn. Then you should kill three of them, run away for awhile, making the mob stop chasing you and wait a few minutes, and the rest of the encounter respawns. That way, you can kill 3 of them at once in a much faster pace.