Using Onager early
First, stand on the railing of a small set of stairs leading down to the landing pad at the very beginning of the section "Keyes". There should be a pipe to the top right, above you. Use the concussion rifle to launch yourself up by shooting at your feet, and land on the pipe. Then, launch yourself up again to the platform with the MAC gun. You can now operate and fire the MAC gun as normal and use it to shoot down the various Banshees and Phantoms flying around.
There is a second method. One can also get to the Onager by jumping on a Phantom as it leaves after dropping troops, and using Armor lock as one hits the death barriers. Then, jump off and bounce off the rock wall. Finally, walk to the Onager. This also causes the player leave the level's map.