Dead Island

This will come naturally as you play through the game. You will first encounter human enemies in Moresby. Human enemies are pretty weak compared to zombies and most the time you can kill them in one hit. The only thing that makes them tough is that they all carry guns so they can hurt you from afar. The best way to deal with them is use guns yourself and aim for headshots. You will find lots of ammo and guns on the bodies of the enemies you kill, you scattered around the buildings they are living in. If you haven't killed 50 by the time you have every other trophy, you can farm this in the villages located in the Jungle. Human enemies will always respawn here. As with every trophy, you can check your progress in the "Extras" => "Trophies" part of the pause menu.