For that level you need to pump anti-aircraft out, but your first move should be to secure the two reactors that are already on the level. You can actually bide your time until you build up a decent force and then start with the objectives since you need to place 5 rhinos while still defending them all. The comp will start off throwing banshees out you with the occasional drop of hunter bond-pair and assorted elites, but increase the frequency of his attacks as you get more rhinos placed.
I built 4 turrets around my base and upped them to take out anti-air units, then built a decent sized force of scorpions, marines, and the tank anti-air unit (whose name escapes me at this point). Used Sgt Forge to protect the one outlying reactor while I used my units to place the rhinos one by one.
Also remember that for the third one you'll need to use a pelican to move the rhino and some marines to the pad. I left some marines there, though they never attacked it again.
Happy Hunting