Kill 4 enemies simultaneously with a single explosion (Campaign only).
You may get this during the story. There are quite a few missions that have lots of enemies in vehicles or you are given explosive weapons to use but as you cannot replay story missions, you will need to get it from elsewhere in Kyrat. There's quite a few places to get this trophy, but the best way to do it is to set off alarms at certain outposts or forts. Firstly, you need a decent explosive weapon. We recommend the RPG-7 as it has good splash damage and fairly decent accuracy. Once you have an explosive weapon head to the fort called "Varshakot" (X:511 Y:431). Once you get there, all you need to do is set off the alarm and then wait for reinforcements. A helicopter carrying 4 enemies will turn up, simply shoot it with an RPG and you'll kill all 4 of them, which will unlock the trophy. Here is a visual walkthrough if you are having trouble.