From Halopedia, the Halo wiki
The player himself must kill all six Wraiths present in the level, as any Wraiths destroyed by any other means do not count, and effectively require the player to redo the level if he or she wants to get the achievement. The exception to this is that the player will get credit if the NPC that killed the wraith is in a vehicle the player is driving. In a co-op game, only the player that got the kill will get credit toward the achievement.
This achievement is represented by a Bronze Cross with a Wraith Tank in front of it.