During the mission "The Heist Begins" you have to retrieve the eDen Spark as one of the objectives. Shortly after this you are tasked with defending an area and various helicopters show up to drop off troops and attack you. Simply equip the Spark and aim, then congrats on unlocking.
If for some reason you don't unlock during this mission there are plenty of opportunities to take out enemy helicopters or "re-oppress" a base if you have already liberated everything.