The Warriors

This trophy will come when you complete the main story on the hard difficulty setting, "Hardcore Soldier"

The game isn't terribly difficult on this difficulty and as long as you are paying attention to your health bar (a colored circle beneath your character), you should be fine. There are a couple of missions where you are better off using stealth and running and hiding from enemies/police whilst trying to complete the objectives as they will spawn infinitely. These missions are "All-City" & "No Permits, No Parley". You should also make sure to revive fallen/un-cuff arrested teammates as a priority as they can help you out pretty well - the game's AI isn't terrible. The enemies gradually get harder to beat and there may be a few times that you will have to restart a checkpoint, but if you are struggling, try to retreat, recover and let the enemies filter towards you wherever possible.