the way i got this achievement was i would play a co-op mode with some fiends online and have the difficulty on hard and alot of the time it might take a couple of tries to do the mission but means this might give you more time to get more heals, but only heal people close to you dont try and run half way across the map to heal some one because the chances are they will die while you are getting to them or you will get downed or killed yourself
hope this helps
o0 JdogJ 0o