It doesn't matter where you start off. Go until you fight the scarabs.

You can just run past the people on the hill or fight though them. Get in a hornet and fly up to where the three towers are .It can be done on any difficulty and any amount people (better done with two people).

When you get up to the towers, just run around until Commander Keyes says "Both scarabs down". Your hornet will disappear.

Then find your way down one at a time. Run down under the bridge you use to go into the citadel.

There will be nothing down there except a few grunts, a brute captain, a gauss hog (there will be marines in the hog), the pelican and a red combat elite with no weapon and no matter what you do, you can't kill him.

I even tried running over and shooting with the gauss hog.