It's a moderately hard level to beat and one of my least favorites. I don't remember my exact strategy, but I think I can help you out.
1.) After killing the elites that were herrassing the civillian center your going to want to move your hawks towards Transport 2 (center one) where there are a few spartans being attacked. Simotaneously your going to want Sgt. Forge going around the map hunting for supply caches.
2.) After clearing out enough covenant with your hawks and gathering supplys from the random containers your going to want to keep Forge handy. After transport 3 is destroyed and the Spartan recomends that you build a base you should have Sgt. Forge waiting in the grass field where one of the bases is available (speed is key). Do not even bother going for that second location Serena recommends as their is a covenant base right next door and it would just be a waste of money.
3.) After you have your base out your priority should be these buildings in the following order: Supply pad, reactor, supply pad, supply pad, barracks (upgrade the first reactor), vehicle depot, reactor 2. Also make sure you have Defensive turrets as the covenant do send drop ships outside your base to attack it.
4.) Make sure that between building up your base your sending out warthogs or infantry when you get the chance to herrass the covenant. I recomend that you send out infantry to those garrisonable lifts that are scattered around the city, as it makes it harder for the covenant to advance on to the transports.
5.) Alright after awhile the covenant are going to start sending a lot of banshee's towards the transports and what sucks is even if you destroy them they kamikaze the Transport taking huge blows to it's health. But hopefully you will have a vehicle depot by then that can spawn wolverines.
Thats the best help I could think of for you. Just make sure you keep sending out units and don't try to tech up right away. Its a hard mission, but I'm sure you can beat it. Speed is key.
Chuck Norris had the cajones to join the Angels of Death....do you? www.theaod.enjin.com
Must be 1000+ years of age to join.