Clear any scenario in Campaign Mode. To make it easy on yourself, do the following:
- Character: Renault
- Class: Renegade
- Mode: Campaign
- Difficulty: Nightmare
Leave rest of the options as it is and start.
* Any character/class should work, but I found Renault's OP shotgun skills made the entire scenario a cakewalk.
Throw away any melee weapons. Pick up guns & ammos. You especially want shotgun and SMG.
Follow the only path possible, killing any zombies you encounter. Try to use the handgun as much as possible in the beginning. Once you pass a car there is going to be a path to the right that you can loop back around. Follow that loop back path and to the left, there will be a red marker. It's a safe that you can open using combination 1077. Take the car keys.
Go back to the original path and keep going where there will be a helicopter on the right, and a car on the left. Use the car keys on the trunk of the car to get wire coil. Use the wire coil on the helicopter. Zombies will swarm in on you. This is where the shotgun is super useful. Use it to mow them all down and go back towards your starting position to finish the scenario.
Video guide here. Campaign mode starts at 3:50: