This video shows the easy way to get this achievement in multiplayer. If you are playing on a low difficulty in single player and are having a hard time getting this achievement then this will help you out. Simple enough, when a squad mate goes down there will be a red cross that appears over their body. Simply go up to them and hold Y to revive. If you are playing on Insanity then you should have no problem getting this during your normal play through. Happy hunting!