Find an ambulance outside any of the hospitals or by killing someone and waiting for one to arrive. Press Up on the d-pad to start the Paramedic mission.
Things to note:
- You must finish each level in one go. You can't stop half way through as it will start from level 1 on your next attempt
- You can't get in a new ambulance if you destroy yours so drive carefully. Leaving the vehicle will cancel the mission
- Pressing Up on the d-pad during the mission will instantly cancel it
- You can carry a maximum of 3 people at once. You can deliver them even if you just have 1 or 2 in the ambulance
Each level contains the same number of people to save. So level 1, has 1 person. Level 12 will have 12 people. At the start of each level, you can pause to look at your map and work out which people can be grouped together to save on time. If you can get 3 at once from one area, try to do that so you don't have to head back there.
The ambulance is top heavy. This means if you go full speed and try to turn a corner, you will probably roll over and crash out of the mission. My best tip is to master using Brake and Handbrake together (Brake: L2 Handbrake: R1). What I do is speed up to a corner, hold brake and handbrake to slow, whilst turning. Usually the handbrake reduces the roll which allows you to make the corners.
My final tip, if possible, try to work on this early in the game. Later into the game, you will have some gangs that want to attack you if you go near them. This makes it harder to complete the mission. I always do it after 1 or 2 missions into the game rather than waiting until later.
Half way through you will unlock a health pick up at your hideout. When you are pretty much finished, you will unlock an adrenaline at your hideout (I had 3 left to save on level 12 when this popped up as unlocked).
When you finish level 12, you will get unlimited sprint (This is another big reason I work on this early in the game) and unlock this trophy.