There are a few ways of doing this achievement, but the 'orthodox' way is to use it on the Wraiths that you encounter immediately after picking up the Target Locator.
As you leave the cover of the base after rearming yourself near the Target Locator and the Armor Lockup Abilities, you will see a Warthog get attacked by two Wraiths. You want to immediately use the Target Locator in the small patch between the Wraiths (replay the checkpoint a few times to get an idea of the Wraiths' movement pattern). The Wraiths will both move to the center to get a view of you, you want to hit them in the middle when they are moving there, so that they wont have time to retreat from the air strikes.
You will probably not get this the first attempt, but luckily the first checkpoint in the map is as you pick up the locator, so you can instantly try again until you land the lucky shot.
Now there are other pairs of vehicles that you could target in other parts of the map, but since these guys appear right after the checkpoint when you pick the weapon up, paired with the slow reaction times of the Wraiths make this the most effective way of grabbing this relatively simple achievement.
Look at this image to see whereabouts you should use the Target Locator:
https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2F...