From Halopedia, the Halo wiki
Wraith Killer is an Achievement in Halo 3: ODST. It is awarded for destroying all Wraiths on the Campaign level Uplift Reserve. It is worth 5 Gamerscore.
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.
- The player will automatically start off with a Spartan Laser. Trade that weapon with a Marine and have that Marine ride shotgun in your Warthog. This Marine combined with your gunner will allow the Wraiths to be destroyed much faster and easier. If the Marine with the Spartan Laser dies, simply pick up the Spartan Laser and employ the same trick again. There is also a Marine already equipped with a Spartan Laser at the second platoon's location.
- Another viable method is to equip a Marine with a Fuel Rod Gun, or any other anti-vehicle weapon.