To start this mission, go to the "W" icon on your map, introducing someone familiar to those that played Far Cry 3. He wants your help landing his plane at the airport, so start heading there. At the airport, Willis will tell you to kill the snipers, but you’ll have to be quiet about it and can’t have alarms being sounded.
There’s a total of six snipers, three of which are on buildings, two in watch towers and one on the hill in the back of the airport. In addition to that, there’s three men stationed on the ground next to the one building, one of which is the commander (yellow marker over his head). There’s also a few other baddies lurking around, one or two near the buildings, and another two blocking one of the road entrances.
If you have the Death From Below skill, then you can take out both snipers in the towers without having to climb all the way up the ladders. For the ones on top of the buildings, each one will have a ladder you can climb to get on top. Do not disturb the commander, as that’s the guy Willis mentioned that he’s meeting with.
Once you have taken out all of the snipers, head to the control tower and wait for further orders. Grab the sniper rifle while watching the deal between Willis and the commander, waiting for your signal to start opening fire (kill the commander first). Willis’ health will displayed at the top of the screen and you will fail if he dies.
When you’re ready to open fire on the commander and his two cronies, wait until the commander lines up in front of the one guy, then shoot with the sniper rifle. This will allow you to get the Two Birds achievement/trophy.
Luckily, his health will regenerate whenever he’s not being hit, so keep an eye on it while looking for more enemies to shoot. Eventually more enemies will come from near your building, and Willis will end up going behind the middle building. This means that you’ll likely need to come off of your sniper perch and get to ground level or another high spot for sniping.
More goons will come down from the mountains, while Willis will move to the top of the warehouse building. Once you’ve killed enough, he will go inside the warehouse and a helicopter will come. It’s your job to now take out the helicopter, either with explosives (there’s a RPG in the warehouse), by shooting the pilot, or just shooting the helicopter until it goes down.
The helicopter has been downed and more enemies will be coming to the party, including a few snipers and one that is manning a mortar. Hiding inside of the warehouse will easily allow you to avoid the above, while easily getting the rest of the enemies to get in your sights without filling you with holes. When the coast is clear, meet up with Willis inside of the warehouse.
After talking to him a bit, with a nod to Far Cry 3, he’ll tell you about a file that’s about to leave on a plane. Once you regain control, you’ll need to rush over to the marked ATV, get on it and start speeding down the runway. As soon as you hit the jump and see the marker to deploy your Wingsuit, do it and don’t touch the controls, except for moving slightly to the right to avoid hitting the back of the plane. You’ll grab the file and the mission will be over when you hit the ground.