Multi-Kill is 4 or more kills at once. You may not get this on your first playthrough, but don’t worry it carries over through each play through. There are several areas that you can attempt this. Chapter 4-5 you take control of the mobile gun battery. After taking control a Iron Whale (transport plane) will come in from the the right. Simply face the hanger door that will open on the side and shoot a shot and destroy all the bots at once, you will get a multi-kill that will flash on the left side of the screen in blue. Now press start and load checkpoint and rinse and repeat until the achievement pops. In Chapter 5-4, when you fight the prototype hollow children (Zombie scrap-heads) you will come to a point where your team opens a door and you must protect them in a narrow hallway, make sure your electric burst capacity and damage, especially damage is upgraded. They will clump together, overcharge your shot and you can score multi-kills. Another method is when you are storming the building near the end of the game, you take control of the big robot with the cannons after Cain hacks into him. Iron Whales will come close to you to drop off some assault robots. Before they drop them off shoot into the side of the iron whale, the door will be open and you get a multi-kill. Killing 4 or more of them, just reload the checkpoint, rinse and repeat.