While in a safe house, preferably one with a combat shotgun, incapacitate the other three members of your group, then revive them. Doing this counts toward your achievement. It is easier with a combat shotgun because it deals a lot of damage fast. In a single player campaign, make sure all other survivors are in the lower red health area. When you incapacitate one of them, the others will try to revive them. You then need to shoot the others before they revive the incapacitated players or you will not get the points for reviving.