The trick to this mission is to keep moving, you can skip most of the opening dialogue (if you know the route of the mission) and keep running towards the first contact with the Covenant. If you're on easy you can run through most of the battle and jump into the vehicle, once they radio in about the stranded marines, you just gotta race over there and defend that area, once that's done you hop in the Falcon and fly to the final area. Once again you can quickly clear this area if you're on easy and just keep killing till Kat closes the door. Race inside for the initial cutscene. After the cutscene, you can pretty much punch your way through the grunts and once you've finished the final elite the mission should be over. I did it in just under 13mins so should be very manageable.