Definitely a case of which way you go, and knowing what you need to kill, and what you can run past.
At the start you do have to clear the courtyard, but on easy isn't too bad.
Then grab the target locator, and hit the sweet spot for both the wraiths, as killing them spawns your warthog.
Once you have you warthog, head RIGHT to the AA gun. Once there, park up by the building to the right of the gun and head straight to the roof, where there's a general on the rooftop. Run right past him and hit the switch, front right corner of the roof.
Then jump straight back in the 'hog, ignoring the 'clear the area' objective and head to the Comms tower. If you carry on round, the first building you arrive at should house the generator, just in and left.
Then head to the roof of the building opposite for the Comms.
Next, head straight back to the gate by finishing your lap, and driving past all the covenant, and hit the button. Run past all the covenant on the ramp/courtyard and head down into where the hunters are.
On your way in, as you see the hunters, there's a shotgun on the pillar in the middle of the ramp, which makes life easier as you have to take out both hunters to progress.
Once in the building just run past everything up to the top floor. There, grab a rocket launcher and take out all the banshees and the phantom.
Par time: 10 mins
I managed to get just under 9, with a bit of faffing here and there.