It's very difficult, but it's NOTHING like Insanity mode. Don't stop for ANYTHING. Just keep driving. If you HAVE to heal, then find cover and do so. Don't get out of the Mako, don't stop to fight, just DRIVE. And make sure to drive in a certian way so that they are less likely to hit you. SERPENTINE! When you get to the Armatures/Colossi, don't stop for them either. Keep going. If you do it just right, you'll be able to JUST make it in time with a little health left in the Mako, not that it matters after that part since you won't be using it again for the rest of the game.