Start a private Zombie Survival mode game. Select the "Max Zombies" setting for more targets. Choose the Battle Rifle and any other desired weapon (shotgun, machine gun, and sniper rifle recommended). You must get a twenty zombie kill streak to get the "Zombicide" achievement. 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. Since the Battle Rifle fires in bursts, a head shot will usually kill a zombie. Body shots will require two bursts. Always try to get head shots, if possible. Use your secondary weapon if you run out of ammunition, which can happen if you get 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, and try again. The zombies will spawn too fast and will overwhelm you.