Personally, I'd say this is easiest using a combination of the Gyrocopter (Buzzer) and M-79 Grenade Launcher.
The grenade launcher is picked up when doing the mini missions to protect an ammo truck heading to a rebel stronghold, so you should get it fairly early on.
I flew over an outpost at high altitude, stopped and used the grenade launcher to kill enemies, allowing reinforcements to come and killing them too. The grenade launcher has high splash damage and takes out trucks, so it's pretty simple. :)
Took me 10 mins to kill off all the enemies via 2 outposts.