Turrets are randomly in certain spots. The Wraiths at the end are sometimes super aggressive so if you get past them (which you can safely) they chase you down the rest of the road to the end really annoying. Marines might destroy the choppers which make the rest of the level easier to do. The end of the mission can be triggered earlier but it doesn't always work. Tried to get better times on other missions. Ended up doing The Ark anyways. Did it in 28 minutes which got me at 2:50:19 for the clear time. :)