For AotCR’s level-specific achievement, you’ll need to keep your tank and kill four Wraiths. Sounds pretty simple, and it is, but if doing it on Legendary you may struggle. If so, as always you can run through again on a lower setting. You need to run past the first one and head straight to the tank, then turn around and wait for the first Wraith to follow you down over the ledge since the tank cannot go back up over it. In a large area with a Wraith camping in an open snowfield, look up onto an upper ledge on the far side from where you enter for another Wraith, making 3 total (this is the one people often miss). The final Wraith is hard to miss in the next canyon area you come to. Handy video for a visual explanation: