High position defence
Use a Banshee to drop yourself somewhere up high, then go to work with a sniper rifle and rocket launcher, or maybe a pistol if you're low enough. The main position I've got in mind here is the first ledge running around the entire area. If you've got Marines somewhere down below, you'll potentially have the challenge of having to work fast to keep them alive. But another way to play things is to leave Marines out of the equation and take your time, just enjoying high-altitude mischief.
Up on the ledge
From the first ledge, this is a great plan that I've had huge fun with, trying to pick off enemies as they charge up the battlefield. It provides possibly the best sniping challenge in the game, what with all those moving targets. And hey, don't forget to rain down a few grenades too! You'll still be down low enough to use a pistol, but you'll probably have more fun with a rocket launcher and it's also good for taking out Hunters (though you can also kill them with several sniper rounds to their blue armour).
When things are quiet, you can also enjoy rocketing distant enemies hanging around near the start of the area. They'll have no clue of the magnificent rockety destruction heading their way. Hee hee! In one case I spotted a wide circle of Grunts and Jackals, way beyond the trough. Well, there's no way I can pass up a target like that. Moments later, a rocket in the centre sent them flying out in all directions. Glorious!
Normally you won't have your Banshee at the end of the carnage. There's hardly anywhere on the first ledge where you can land without it sliding off; and you have to be fairly careful not to slide off yourself when you land. Amazingly though, it's possible to jump down safely to congratulate the survivors or to go in search of any remaining covie dirtbags. If you get it just right, the sloping cliff base breaks your fall and you don't lose a single bar of health!
…with pen at back left
Here's one particularly good configuration you should try. Have the Marines in a pen towards the back left, e.g. based on the trees at back left. Fly straight past and land up on the first ledge a short way back, near the corner. Your Banshee will probably slide off, but you should be ok if you're careful. You get a great view of the enemy charging up the field. They're being drawn your way as usual, but they'll get waylaid by the Marines of course. It's quite a spectacle, and it's plenty of fun to simply watch if you want.
In the save I was using for this (see picture), I also had Sergeant Johnson ready on a chain-gun off to the side; which was just as well because although I had eleven guys in the pen, they needed help. A chain-gun really adds to the spectacle. And don't you just love that sound?
You can leave the Marines penned or release them. When releasing them, I found that lots of enemies tended to come up the inner platform of the pipe structure, like they do in the platform ambush plan. In fact, it was basically that same situation, except that I was doing my ambushing from a long way off. For example, I was able to time rockets so they blasted enemies just as they came to the end of the platform, towards the waiting Marines.
Mischief from the bridge
You could even drop yourself off on one of the bridges to have some mischief from there. Peek over and enjoy sending down rockets and grenades, and watching the delightful results with a sniper rifle. You can also create some fine chain reactions by sniping some Elites or Grunts first, to get lots of plasma grenades littering an area.
Overhead pipes
You can wreak a bit of overhead carnage by landing on one of the large pipes going across the central structure. Although it's a bit awkward, there are places where you can avoid most enemy fire, and where you can get sneaky shots on the enemy. Explore.