Did this in 5 minutes 27 seconds.
To start the level use the warthog and just drive. You can drive it over the first barrier in the tunnel.
The next thing is getting into the gauss hog. Take out the first sniper, drop down and get to him, avoid the 2nd sniper from the balcony. Drive past and proceed.
You will face 2 wraiths and 2 ghosts before the door will open. Drive around and survive.
Next is the scarab.
You can proceed up when the doors open, go to the next door. when the scarab is to your left the door will open.
grab the rockets and proceed to kill everyone on the scarab which is mission end.
If with any luck like me you will also get the achievement for Not dying on Normal.
Good luck !