Successfully complete the game, then enter the options screen and save your profile; the elite difficulty setting will now be unlocked.
When confronting guards in almost any stage of the game, headshots always work best. They kill in one shot and leave the body armor undamaged. If you need to use multiple bullets, the body armor collected after searching the body will be damaged and less useful.
The health pack (default is Left) never runs out. This is basically the same as having infinite health. If you are ever injured, no matter how badly, use the health pack. The only downfall is that you need to be alone, as it takes some time to actually heal. After a firefight, find a secluded room or corner and press Left.