Partime is 15 minutes, practice makes perfect. Below are some benchmarks for each stage of the mission. if your timer goes beyond them either restart or use the rest of the run to familiarise yourself with the level.
On the Mission Select Screen; Make Sure that the Time HUD is turned on (Extremely helpful), Switch the difficulty to easy, turn off Skulls both scoring and non scoring skulls.
If you restart a checkpoint your time continues on as normal and will not reset.
When starting the level, run down into a courtyard with a tiered balcony. Waves of enemies will come. Stay central, listen to gunfire and kill off any enemies ASAP.Eventually Two Hunters Will Smash down a Double Door. Aim for there backs and throw grenades to quickly take them down.
After Killing the Hunters Your time should be around 4-5 Mins.
Head through the door the Hunters Smashed down. Run through the streets, shoot at enemies but dont waste time stopping, or trying to kill them all.
You will come to a Hotel, and a soldier telling you to follow him.
Your time should be around 6-7 mins
Run through the hotel and you will come out at the beach.
Jump down and you will see a ghost next to a crashed landing site.
Ignore the Warthogs, the Ghost is your best friend in this level as you can boost Jump in the Ghost and boost all the way round the beach, again, avoid enemies. Eventually you will come to Tunnel, (A few yards from the tunnel is a stone hut which contains a terminal).
Your time entering the tunnel should be under 10 Minutes. If your timer is around 13 Minutes or over just restart. If you are at the 12 Mins you may make it but wont have much room for error.
Boost your ghost all the way round the tunnel, avoiding stopping for combat,and it should only take around 2 minutes.
The tunnel section can be very long in the Warthog, it is less maneuverable and much slower than the Ghost.
Practice makes perfect, if you go over those benchmark times i gave don't give up, learn the rest of the area. It is unlikely you will get this first time, but hopefully this willgive you a few tips to help you get it faster
EDIT: Jobbenar has commented that the fight with the Hunters can be avoided by running past them, can anyone else confirm this? I thought killing them was neccessary