Something important to remember is that killing marines is faster than rescuing them. This is best done on Easy as combat will be a breeze. Also remember it is best to not use any skulls.
I take no credit for this video, but it shows the best path to take for the achievement and the methods you should use. I will also provide some annotations.
The grenade jump at the beginning is pretty unnecessary and is a lot more dangerous than it is worth. The only positive effect it has is to despawn enemies but if you are on Easy that isn't a problem.
In the first area with marines you fight 5 waves of enemies, all coming from dropships at various points in the area. In the video he uses lots of grenades for these waves, but I preferred to save them for later in the level since you have to use grenades to kill the marines or they will become hostile. One last note, killing the marines in the first area only skips dialogue and saves minimal time.
Once you get in the warthog and have to drive to the area where you activate the light bridge, you can skip the cutscene which shows you hitting a button and the bridge appearing by taking the path shown in the video. Most people will want to replace the fling jump with simply jumping on the turret of your warthog and then jumping again to land on the incline.
I would highly recommend going in the order the video does as far as crash areas. Remember to make sure you have killed all the marines before leaving or you will have to come back. A good check is once you have killed all the marines in the rocky area assuming you also got the underground marines, an objective will appear on the screen telling you to "Search for the last Survivors". This means you are okay to proceed to the final area.
This is probably one of the more difficult levels to get under par time, but it is very easily doable if you follow the methods I outlined here. Comment or PM me if you have any questions. Good luck and don't forget to go fast.