Actually, you can easily accomplish this by just building Vultures.. They are available on that map for a good damn reason.. I used only THREE vehicles to complete that map.Hmm I'll admit that level with the timer was a bit challenging, and i had to play it on the easiest difficulty. All you need to do is pull a massive Scorpion tank rush and then use them to guard the Flood spawns from regenerating, then retrieve and pull your units in from across the map.
Just create three Vultures and upgrade to Mega-Barrage... What you do is do a hit and run tactic on the things that spit at your planes.. You select it to Barrage one, then once it lets the missiles loose you get out of range of the others. You use each Vulture independently on each thing that attacks you. They gain veterancy damn easy as well, at the end all three had two or three stars. For the flying units, you use Cryobomb and Barrage those as well..
L2Play this easy RTS.
Edit: I did this on Legendary easily.. It was so much easier than on Easy.. I mean it, once you learned how to play.