You will earn a medal for lasting a certain amount of time on each campaign level. Pick up only melee weapons so you don't use a gun by mistake.
Having two AI players and two human players will work if you can't find three other players to do this. Load up any of the levels and have one person hide in a corner. Stick together and have the rest of your team protect you.
Bronze Medal: 4 Minutes
Silver Medal: 7 Minutes
If you hit four minutes and fired a gun by mistake you will have to either survive to the next medal, or choose another survival level.