Extract 2 snipers at the Mountain Relay Base
The snipers that you seek are in Mountain Relay base. Extracting them is an optional objective in this mission and it is best to do this when going to the caves. After reaching the base, use the binoculars and try to identify the snipers based on the weapons that they carry. They should be located on the road or right next to it, one to the left and one to the right of the bridge.
The easiest way to get close to them is to go under the bridge and then use the stairs or climb up the rocks. Each one of them can be dragged away from the rest, or you can neutralize all the enemies in the area. You should of course Fulton both the snipers.
Eliminate the gunship
The enemy gunship is patrolling the eastern part of the map, namely the area near Da Smasei Laman and outposts number 14 and 17. To shoot it down, you will need an appropriate weapon. You can unlock it by research in Mother Base. It can be for example grenade launcher FAKEL-46 or rocket launcher GROM-11.
If you have the rocket launcher, you can attack the gunship as soon as it appears in your range of view. If you use the grenade launcher, wait until it stops in one place and quickly get as close to it as you can (avoid the searchlight). Shoot a burst at the machine. 3 hits from the grenade launcher or 1-2 from the rocket launcher should be enough.
Note - when the gunship is damage, its pilot may decide to try to escape. Don't allow it, because when the machine leaves the mission area, you won't be able to complete this objective. In such case, it is good to load the last checkpoint and try one more time (shoot one missile after another, without any additional breaks).