Here are some general remarks about twin bridges megabattling - mostly with Marine support in mind.
With such a large area to play with, there's huge potential for creating different battles, depending on how you start the enemy off and what you do with your Marines and yourself.
Typical battle flow
Typically you'll be landing somewhere towards the back half of the area to defend. A mass of enemies will head your way, invariably led by Elites, but many others may stay behind. Whether that's because they lost track of you or they figure they're not really needed, I'm not sure. Perhaps it's mostly the latter, because they tend to be spurred into action later as you decimate the first wave, as if they sense that reinforcements are needed. Some free-roamers may still remain behind though, and only spring to life when you approach them later. Usually they'd be right near the start of the area, often out of sight down the slope towards the zigzag path. It can be quite dramatic when a mass of them come into view over a hill and you find yourself under attack all over again, just when you thought you'd cleaned the place out.
Tweaking for different experiences
Bear in mind that you can tweak things before taking up your defence position, to make for significantly different battle experiences. In particular, a bit of flying around can adjust the enemy layout to influence their eventual attack. See the section on Getting variations of a save for elaboration on such matters. That section was concerned with also getting a checkpoint to moreover make a replayable variation; but obviously you don't have to bother with a new checkpoint if you don't want to.
Giving yourself a chance
On Heroic (my preferred level for this battle), a heavy enemy charge led by Elites is quite survivable on your own or with a small squad, if you open your defence with some efficient sniping. Surviving a heavy charge without a sniper rifle is going to be a lot harder if you don't have much support. If you want to battle using shorter range weapons right from the start (for more visceral action), you can give yourself more of a chance by having support from a chain-gunner or sniper, or a larger squad, or by limiting the number of Elites present. I've typically got a large squad and often a chain-gunner. I rarely have a sniper though; see Sniper support for discussion.
With the high enemy numbers you can get in the battle, a degree of framerate slowdown is only to be expected. However, on Heroic I'm often fighting more than fifty enemies with a dozen or more Marines, and it's usually fine, though the trough defence seems particularly susceptible to giving slowdown with so many Marines. If the degree of slowdown troubles you too much in a battle, you'd best limit yourself to fewer personnel in future. Eliminating territorial enemies is one idea. Limiting the Marine count to perhaps no more than eight is another. Sometime I'd like to do some testing to try to get a better handle on this. Maybe later.
Hey, I wanted that!
With a highly populated battlefield, the game can be very quick to remove bodies and dropped weapons as soon as they're out of your field of view, to alleviate its workload. Because of this, a weapon can disappear just as you're approaching to pick it up; namely when it goes out of sight off the bottom of the screen. Quite annoying, especially if you were counting on it to fend off the next couple of Elites heading your way! To stop a weapon or grenade being snatched away from you like that, make a point of looking down to keep it in view until picked up.
Cortana to fire team Zulu
When you've wiped out the enemy (including the door Elites) and you're with Marines, you sometimes hear Cortana say "Cortana to fire team Zulu. I've sent a distress call tagged with your current position. Hold your position and await evac. The Master Chief and I are going to continue on ahead." But not always. I'm not sure exactly what determines whether you hear it or not.
Without a doubt, battling alongside a large squad is the highlight of this megabattle, and for me it's a major highlight of the game itself. The close-quarter combat is simply fantastic and the scenes of carnage and mayhem are like nothing I've seen before. The spiral path megabattle was hectic enough, but this takes things to a whole new level, thanks in no small measure to the wide open spaces that let the enemy and Marines move around so freely, and that give you so much potential for setting up different encounters.
Effective Marines… at last!
Part of the difference is that in large numbers, standard (non-sniper) Marines become surprisingly effective. Their individually feeble efforts mount up to quite a hail of fire, which I think causes enemies to spend more time dodging around and getting flustered, thereby reducing the amount of return fire. It's very much a case of the whole being more than the sum of its parts. On Heroic, a squad of four standard Marines contributes little more than atmosphere and distraction, and you'd barely trust them to take care of a Grunt with a paper bag stuck on his head. Yet a squad of eight is actually a force to be reckoned with, and by the time you get to twelve, things are really cooking! It's a radically different battle experience from normal. Another nice thing is that when you need to recharge your shield, you can do so by backing off to let the Marines take the brunt of the enemy attack for a while, even with marauding Elites present.
A sniper's dream
The battle is an absolute dream for sniper rifle enthusiasts; easily the richest situation in the game. Lots of distant enemies, both moving and stationary, and you can tackle them from ground level or from up on a ledge. You could even have a spare rifle handy for extra ammo. Often you'll have a big angry mass charging up the field at you, and the ability to drop Elites with rapid headshots can mean the difference between life and death. With so many enemies heading your way, you can often get two or more with one shot, something worth looking out for.
Even if you do a good job sniping the Elites though, there's still a swarm of lesser enemies to worry about, and potentially Hunters too. You'll probably have to periodically break off from Elite sniping to take care of other enemies that have become too close for comfort. Unless you've got a large squad, you can't really trust supporting Marines to do that, as they're pretty hopeless at putting enemies down. A pistol is surely the best thing to have for this. It's also useful for finishing off an Elite after not quite doing the job with a sniper round. Also be ready with frag grenades, for when you see a nice concentration of enemies.
With a Marine-retaining Warthog on hand, you can get chain-gun support if you want it. You can leave it manned and ready for battle. Perhaps stationed to one side of a pen, or behind a blockade. It could even be part of the pen or blockade. Think about the positioning; you don't want the gunner to end up spending most of his time shooting friendlies in the back! Putting it on raised ground can help. If you want to weaken the support to make the battle more challenging, try placing the hog where it'll be further away from the advancing enemy.
Dual chain-gun fun
Maybe arrange a second hog that you can jump into yourself, for a bit of dual chain-gun fun when you feel like it. You could be on the opposite side of your pen or blockade, to form a lethal crossfire. Or simply put your hog alongside the first so you can enjoy the full aural glory of dual chain-guns rattling away.
Not today thanks
If you've set up a gunner but decide that you don't want his help on your current play of the battle, a quick way of ejecting him is to send in a carefully aimed Banshee blast as you arrive. I'm often doing that; sometimes I want all my troops to follow me to a completely different part of the battlefield.
Stacker and Johnson
You can have a chain-gun manned by Stacker or Johnson if you've used the method of getting seven extra Marines at the start. They've got the best vocals, and Stacker is a real hoot. See Extra Marines for related talk.
You could have up to two snipers in the battle, but whether you want them is another matter.
Personally I don't usually like to bring snipers into the battle. For one thing they can remove too much of the challenge, being so lethal. For another, they're a danger to the rest of the squad. But most of all, it's annoying when a sniper kills an enemy you were already taking care of, robbing you of the satisfaction and causing you to've wasted time and ammo. That's particularly the case when you've been busy piling needles into an enemy and looking forward to their agonized explosive demise, which is one of my favourite pastimes. Obviously they'd be a great help against a horde of charging Elites, but I'd usually rather balance that challenge by having a large squad and maybe chain-gun support; or limiting enemy numbers if necessary.
But then again…
On the other hand though, it does make a nice change to have the luxury of a sniper after taking heavy punishment in a lot of hard-fought battles. It radically alters the balance, and it can be fun to just watch the carnage he wreaks (maybe watching advancing enemies with your own sniper rifle as he takes them down). Also fun to listen to his continual smug comments. "Didn't even see me!". His long-range ability frees you up to carry shorter range weapons, and you can enjoy stealing a few of his kills, finishing off Elites with fast pistol work or whatever. In fact, things can get quite a nice teamwork vibe. And lets face it, it's a pleasant novelty to be fighting alongside a Marine who can actually do some serious damage!
A sniper can also offer long-range support if confined by a pen or blockade, or seated in a Marine-retaining hog. For example, I once had a save that included Sergeant Johnson in a pen at the back of the area, armed with a sniper rifle. Sometimes I tried to hold off the enemy from near the trees past the door. They come charging up the side, and Johnson gave me a helping hand by picking off the occasional Elite from way over yonder. Nice one Sarge! That was ideal: he was supportive without being too dominant.
If you put a sniper in the seat of a Marine-retaining hog, you've now got a way of placing sniper support wherever you want. It's also a way of keeping him separate from the main crowd, to prevent him shooting friendlies. You can stick him off to one side of your defence position or something.
Mind you, it could be tricky to get him into the seat in the first place, when lots of other Marines are around, all happy to jump in. So here's an idea. When creating your base save, get a sniper seated as early as you can, and leave him there. When creating a battle, you can always make him dismount if you want.
Newsflash! In a late breaking development, I now realize that you can get a sniper up onto either of the pipe blocks - the huge long blocks that support the greenish metal pipes across the trough. From there he could be in a great supporting position, hopefully posing no danger to the rest of the squad. To get him up, first get him in a hog and park so his side of the hog is against the block. Make him dismount. He'll probably end up part way between the seat and the block, a few feet off the ground. If you now fly at him in a Banshee, you can make him dive onto the block. You can then move the hog clear if desired (perhaps by blasting it, to avoid any possibility of him trying to hop in). He should stay in place, waiting for battle.
Hunters can be a bit of a pain in battle, sending in long-range blasts that your Marines are hopeless at avoiding, quite apart from the danger and distraction to yourself. You can always omit them from the battle of course, but they do make for a good challenge, and it's great fun to rocket them from long distance - either aiming for the ground nearby or, my favourite, letting 'em have it square in the face. They can also be distantly taken down with several sniper rounds to their blue armour, but that's nowhere near as satisfying.
However, if you've got a well advanced blockade, I suggest you omit Hunters from the battle else they'll tend to blast it pretty quick, putting a damper on your fun when trying to defend from behind it.