You might get lucky during a combat scene and come across 4 enemies close to eachother that you can quickly run over in under 20 seconds. Since enemies can run around the place as they please, you may or may not get certain opportunities to get this trophy at different times. It isn't too hard, so you will get it eventually.
If you are having difficulty, the video guide below will show you a reliable tactic for gettng this trophy. At the end of chapter 3, you will see 2 enemies walking in the water. Run them down with your car and then drive up the hill to find 2 other enemies to run over to get the trophy.