ummm... get the rocket first, doy.
send a guy racing off in the ghost FIRST thing to get it, give him support with the banshee and have a sniper keeping tabs on things, meanwhile, the other guy(considering this is a 4 person CTF) get the BR/Plasma pistol combo and hops in the warthog, by now, you have the rocket and killed some of their people/vehicles in the middle, or you suck. Have the BR guy drive the sniper and warthog to one of the hills and drop the sniper off so he has a vantage point for sniping. pick up the rocket guy in gunner and banshee drives off, have banshee guy kill anyone on top of base and drop onto it. nade spam the flag area and by now have the underway of the base cleared from hog. Banshee guy drops in a kills/wreaks havoc as much as possible, while the rocket guy jumps on top of hog and grabs flag through floor, killing anyone who stop him. Sniper covers, and you drive home.