First, it is recommended to unlock the Non-Lethal Assault Rifle Lv. 1. You get it automatically from Story Episode 4 (Order No. 41). This cannot be missed. Go to a MULE Camp in the Central Region and stun all the enemies. MULE camps are marked on the world map by orange outlines. They contain enemies that try to steal your cargo and hunt you.
To get the trophy, you must stun all enemies in a single camp. When done correctly, there will also be a text prompt in the bottom left that says “Porters will temporarily be able to traverse this area.” Make sure you really stun every single one of them. Use the scanner to locate the enemies when they are hiding. The camp just south of Lake Knot City is recommended for this. It has very few enemies and no reinforcements driving around on trucks. The reason you should use the Assault Rifle is because it makes knockouts fairly easy. The Bola Gun is not recommended as it only traps soldiers with a rope (does not really “stun” them) and they break free after a few seconds. Alternatively, ramming them with vehicles (truck/trike) works and will instantly stun them. Alternatively, you can use your bare hands to strike them or throw cargo for instant knockouts. Also, if you use the Non-Lethal Assault Rifle, make sure to switch to rubber bullets. Hold Right to open the weapon wheel and press Triangle on the weapon to switch ammo type. Note: You can change the difficulty to “Very Easy” in the game options to take more damage and stun the enemies more quickly.