Kill 30 enemies with an elephant (Campaign only).
First things first, you need the "Elephant Rider" skill. This is available from the get go and it costs 2 skill points. You might get this just by playing the story, as there are multiple chances to ride an elephant in story missions. If not, you can farm it at the "Shanath Breeders" Outpost (X:380 Y:568). There is an elephant in a pen at this outpost, and you can simply run straight to the pen and get on him by pressing . Now just ride it around the outpost and knock into enemies. The elephant will one hit kill everything. When he cars turn up, you can press and the elephant will flip the car and everybody in it will die. There's about 10-15 enemies in this outpost, so it will not take long. Once you have cleared it, simply go to the "Outpost Master" mission found within the Outpost to replay it. Keep doing this until the trophy unlocks. For a visual walkthrough, refer to the video below.