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.