There are a total of 24 outposts ranging in difficulty (easy - medium - hard). You can discover all of them by hacking the bell towers.
The best method for all outposts is using a silenced sniper from a distance. Here are some useful tips.
- Use the camera
to tag enemies and locate the alarm boxes.
- Shoot the alarm boxes when enemies aren't near them. They can be alerted if they witness the alarm box being destroyed.
- Free locked animals by shooting the lock off a cage. They can take out most the enemies.
- Watch out for mortars by the icon that appears of your screen.
- Go for headshots which kill all human enemies in 1 hit including heavy gunners.
- For the harder outposts keep an RPG or grenade launcher on you to take out helicopters if reinforcements get called in.
- Manually save before approaching the outpost so you don't have to travel there again if you fail.
- Monitor the detection meter, if it starts to fill quickly crouch or run to a form of cover (trees, bushes etc).
- Once the final enemy has been killed the screen flashes a colour and switches to slow motion.