Start a private Zombie Survival mode game. Use the "Max Zombies" setting for more targets. Select the Battle Rifle and any other weapon of your choice (shotgun, machine gun, sniper rifle recommended). You must get a twenty zombie kill streak. This is only possible if you kill zombies quickly between each other. Do not waste shots. You will have enough time to reload between kills; do not waste time by killing a zombie after a reload. Because the Battle Rifle fires in bursts, a head shot will usually kill a zombie. Body shots will require two bursts. Always try for head shots if possible. Use your secondary weapon if you run out of ammunition, which can happen if you try too many body shots. Do not scope in with the Battle Rifle because zombies can approach from multiple directions and your peripheral vision will be narrowed. Grenade kills will not increment the counter. If you die, end the game instead of respawning. The zombies will spawn too fast and will overwhelm you.