In the third mission "Persona Non Grata" you'll follow Price into a cellar and be presented with UGV. For this achievement you'll need to take out a total of 5 helicopters with the UGVs grenade launcher.

You'll have a total of 20 grenades on the UVG, and each helicopter takes 2 grenades until they're destroyed. Your best bet here is to take it slow. Pressing RT But will shoot the minigun, so use this on all the ground units. It has unlimited ammo so don't worry about running out. As soon as you spot a helicopter, let it take its position and take your time. Once you have it in your sights press RB But to fire one of the UVGs grenades.

Here's the order they come in:

First one is right outside of the garage from where you take control of the UVG

As you make your way onto the construction building, the 2nd will fly up from the right
After the 2nd helicopter, 2 more will fly over the hills to your left
The last one will fly from right in front of you as you get to the second construction yard.