Encounter: Ruins (Chapter 8)
Select the Ruins encounter from the main menu (chapter 8). This is a very good place early in the game to get the trophy done.
Play on a low difficulty. Always kill one enemy with gunfire, then one with melee attacks and repeat. You need 10 kills total (5 gunfire, 5 melee). All the enemies in this area are really weak so it should be easy. Use Melee kills on the enemies on the ground and gunfire on the ones on the ruins. You can't reach the enemies on the ruins, so you must reserve your weapon kills for them. If you shoot too many on the ground you might run out of enemies.