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Base your defence on the trough running up the centre of the battlefield. There are lots of carnage-heavy possibilities here, in view of how the enemy pour into the confined area.
Defending from the back end of the trough (i.e. furthest from the enemy) is a whole lot of fun. I've sometimes got spare weapons there ready for this, including a rocket launcher. The enemy will charge into the trough from the other end, though you may also get some coming along the platforms (unless you block them off with a few Ghosts or something). Initially you'll probably want to do some sniping, but rocketing is also a fine idea, especially if you see any enemy concentrations forming.
If you've got Marines, they tend to rush down into the trough to mix it with the enemy. Not a good idea guys! Still, it does give you the challenge of trying to keep the idiots alive. When the enemy start to get closer to you, a plasma rifle is probably the best weapon to have, perhaps coupled with a pistol. When Elites are charging up, a stream of plasma to the face is good for stopping them in their tracks. Only trouble is, there are probably another two or three big nasties just behind! Oh dear.
Close and nasty
If you prefer, you could go down into the trough for a more close-up affair that's guaranteed to be even more hectic. This probably works best if you can get down there with a large squad of Marines before there are too many enemies on the scene. You'll have more of a chance of holding the enemy back, though on Heroic it sure won't be easy. The scenes of carnage down there can be quite amazing. If you've got a sniper, he tends to stay back near the top of the trough, picking off enemies for you. Nice! If you need to recharge, duck into one of the side passages.
If you go along one of the inner platforms running alongside the trough, the enemy doesn't seem to come after you directly. They mostly seem to get drawn into the trough, making some pretty fine targets. Marines get drawn in too. You'll have to watch out for enemy fire but it's still a bit like shooting rats in a barrel. Rocket launcher, anyone?
Some of the best trough battling can be had with a blockade across it, serving to either keep the Marines back or the enemy back, depending on how you play it. It makes things rather less chaotic. See Trough blockades for set-up advice. I'll talk about some battle specifics below.
Blockade just past the middle
With a blockade just past the middle of the trough (i.e. slightly towards the enemy's end), the sniper rifle and rocket launcher make a great combo. From your low perspective you'll be seeing waves of enemy coming at you and they make a wonderful target for the launcher; you can hardly miss! Sometimes I fire a rocket even before the enemy are in sight, banking on the expectation that a mass of them are just about to come over the rise (you soon get a good idea of how long it takes for them to appear). And BLAM! They run straight into an early surprise. Pretty funny when it comes off - and let's face it, the sooner you start decimating the advancing horde the better. You can also be sniping Elite heads the moment they come into view.
Another combo I like for this defence is the pistol and plasma rifle as they're both fast-acting. But I'll usually be shovelling lots of grenades too. Further into the battle, you tend to get groups of Jackals crouched in support of more advanced colleagues. The glowing shields make such groups very obvious, and they're great to rocket or grenade.
I've been quite impressed by how far a Grunt is able to lob a grenade at the Marines by the blockade. They can do it from way back up the slope, and it's something you'll probably want to watch out for to try to prevent. Something else to watch out for is enemies coming along the platforms and eventually attacking you from behind, which could be a nasty surprise. One way of countering that is to have a chain-gunner standing guard - for example at the end of the trough. Alternatively, you could've blocked off the platforms when forming your megabattle save, though you could still get enemies going right around a pipe block.
Platform support with a blockade
With a manned blockade just past the middle of the trough or even more advanced, try stationing yourself up on one of the side platforms, or even on top of one of the passage arches (hop down onto it). As long as the Marines are ahead of you, I find that the enemy focuses most of their fire on them. Meanwhile you can create havoc from on high with your methods of choice (and you don't have to stay back; you should be able to make brief advances to attack from close range if you want, peering down onto enemy concentrations). Great fun, especially with a rocket launcher. With this plan you'll probably want to've blocked off the platforms to stop enemies coming along and troubling you.
You can also adapt this plan to the case where your blockade is well back near the end of the trough. From your platform, let enemies run past you below and attack the Marines. Now send in rockets from behind them, or whatever. Nasty surprise for those Elites!
Distant support with a blockade
With a manned blockade near the middle of the trough or more advanced, you can support Marines from distance - probably using a combination of the sniper rifle, rocket launcher and pistol (and maybe a few long-range grenades). Even if you're right back at the end of the trough, the Marines will still be inclined to remain in the trough - you have to be further back before they'll withdraw to flock after you.