I just completed this last night -- FINALLY!
First things first: make sure you are taking full advantage of game save feature -- i.e. once you get your base underway save so you can quickly get back to that point.
As noted previously, air (and some decent micro) is the key to success on this mission. Focus on getting that quickly above all else. Once you get to about 6 or more Hornets you should be able to control the game. Try positioning them near the ramp that leads down into the are where the second base and Cov mega turret is. That allows you to deal with the Wraiths and Hunters (or Ghosts). Your Spartans and other infantry should be garrisoned in the downed pelican positions on the slope into the crater. They can handle any infantry from there, they just need your Hornets to keep the Wraiths and Hunters off their backs.
It's then a matter of holding out until Spartan Omega arrives. Once they do you have won the mission. You roll them and your other forces over to the mega turret and take care of business. From there I built up my second base maxed out on Hornets, and then sent everything I had to quickly take down the base.
Once I eliminated the mega turret and built that second base, not so much as a Ghost attacked the crater.