Associated movies… BCM18
In this favourite plan of mine, enemies get drawn along the inner platform that runs along the side of the trough nearest the door, and you attack them at the end, potentially ambushing them at close range.
The key to making them head up that platform lies in your positioning. You need to be roughly in line with the pipe block or slightly to the right, in line with the inner platform. If you're too far right or if you go along the platform a little, they'll be drawn into the trough and so will Marines, while if you're too far left, they'll tend to come around the outside of the block. Stand within those two extremes however, and most or all enemies will trail along the inner platform as desired, feeding them straight into your sights.
You can be waiting unseen, hidden by the platform block and ready for close-range action (which is when the 'ambush' aspect is at its strongest), or you could stand further back. If you've got Marines they'll be standing quite a way back and it's great fun to join them.
Let 'em have it!
If you're ambushing from close range (just off to the side), one great combo is a plasma rifle plus AR. You can have a lot of fun drilling Elites with bullets or plasma as they appear. If you've got Marines, Elites will often be galloping straight towards them, not even noticing you at first. There's great opportunity for melee if you're keen on that. I like finishing off Elites with a whack after giving them a good drilling - and it saves a bit of ammo too. The intimate situation is also ideal for doing 'tag and dodge' moves when Elites take a swing at you. I quite like deliberately provoking swings so I can do that.
A rocket launcher is another fine weapon option. That can give arriving enemies a real nasty surprise, as you can see in BCM18. It's rather satisfying to blast Elites at close range from the side, just as they think they're about to start slaughtering my Marines.
The needler is glorious when used on enemies still advancing up the platform. One after another you can send them up in magnificent pink explosions, often followed by a chain reaction from all the grenades that have built up. Needler plus AR is one of my favourite combos. I'll take out as many Elites as I can with needle explosions - maybe some lesser enemies too - and the AR comes into its own when things get hectic close up. You can usually keep a needler well supplied with ammo by making sure to let occasional needler-wielding enemies get past the end of the platform. Kill them and their ammo is yours. See how many enemies you can kill with just your needler; that can make for quite a skillful challenge I find. Even Jackals can be sent up with a bang, by slightly moving to the left as you fire. They'll turn a little to track you, which allows needles to sneak in past their shields.
You can also wreak havoc with grenades of course. You can plop grenades down ready to explode just as enemies are reaching the end (watch your motion tracker and use your ears), or you can stick your head out and lob them down the platform, often causing enemies to comically dive off into the trough. See those Grunts go! I swear those things on their backs double as stabilizing fins for grunty flight. Elites sometimes jump clean over to the other side! You can also throw grenades down the platform without even exposing yourself; just throw them 'blind' over the corner of the pipe block. With enemies flowing thick and fast, you're in with a good chance of blasting something. Another bit of fun is to anticipate the arrival of an Elite or Grunt and try and throw a plasma grenade to intercept him just as he arrives.
You can get a succession of great chain reactions at the end of the platform, as grenades build up for a while then get detonated. In fact, you can encourage that by deliberately killing enemies non-explosively for a while, then picking your moment for a massive bang that takes out some arriving victims.
Handy recharging and rear attack spot
Sometimes when out front attacking enemies from the side, you may misjudge things and bite off more than you can chew. Maybe you'll have a nasty experience with multiple red Elites turning up at once or something. If you need to recharge, you can potentially back off a few metres to hide on the outer side of the pipe block, letting the Marines deal with things for a moment. Handy!
From this spot you can also attack enemies from behind while they're focused on the Marines, which can be quite good fun if you've got a suitably resilient squad. Give it a try. A pistol is an excellent choice for precision shooting to avoid damaging your Marines, but a sniper rifle and plasma rifle are also good. The needler is plenty of fun if you don't mind causing a lot of collateral damage from the resulting explosions.
Watch those approaches
If you do back off around the corner for cover, preferably stay close to the block. If you stray too far left (like, more than halfway across the width the the outer platform), that will tend to draw distant enemies around the outside of the pipe block; so you'll have to watch out for that. Similarly, moving too far right (or advancing a short way along the inner platform) will tend to make enemies start coming along the trough. Even if you don't do either of these things, some enemies could still come around the outside or along the trough or even on the far platform. That just depends on where they were before you got into position to draw enemies along the platform. So watch out and be ready for them. If you want to minimize any likelihood of enemies approaching via other routes, fly straight to your defence area.
If you actually want some enemies coming along the far platform (it's fun to rocket them, and sniping would be another option), that's fairly easy to encourage when you fly into the battlefield. Instead of heading straight for your landing spot, deviate for a while to draw enemies that way. The longer you spend drawing enemies over to that side, the more enemies are likely to use that far platform. Experiment and you should soon get a feel for this.
You could maybe have a gunner in a Marine-retaining hog for support. One nice position for the hog is to have it backed up onto slightly raised ground against the cliff, at the back left. Being on raised ground makes the gunner less likely to shoot friendlies in the back; hopefully most of his fire goes over their heads. Another idea is to station yourself back with the Marines and have a gunner well forward but off to the left, so he'll be firing into the enemy's side as they appear. Nice bit of crossfire! I'm sure the covies will appreciate that.
Up on the pipe block
One other thing to consider doing at some point: you can jump up onto the pipe block and attack enemies from there. You could even shelter around the outside of the first pipe to attack enemies from behind while they're focused on the Marines, much like I earlier mentioned for the outside of the pipe block. Plenty of scope for amusement here. Explore the possibilities. You'll need to keep to the outer side of the block to keep enemies coming along the platform. If you stray too far towards the platform, they'll start heading into the trough instead.
Fantastic long-range defence too!
You can have a superb time executing a long-range defence against a steady stream of enemies coming up the platform. Position yourself so you've got a view right down the platform, and you'll have a ball. It's not exactly an ambush, but it's the same enemy approach dynamic going on. The covies make fantastic targets, and with a sniper rifle you'll often be able to kill more than one at once. See how many you can manage. A sniper rifle and pistol make a great combo here, and you can enjoy seeing how clinically you can wipe out the attack. Just as with the close-range ambush, there's also great potential for chain reactions - this time set off by either a rocket or grenade.
For this plan you won't want Marines running about in front of you. So if you've got some in a pen, maybe leave them there. Alternatively, make sure to stand well to the fore. Incidentally, I'm not just thinking of the sniper rifle, pistol and rocket launcher here. Don't forget that the plasma pistol has long range too. You can potentially wipe out the entire attack with one of those.
As already mentioned, enemies start getting drawn into the trough if you move too far right or if you go down the platform a little. Likewise they may start getting drawn around the outside of the block if you move too far left. This is really no problem in practice, once you get a feel for the area you need to remain in. However, you could prevent the enemy taking such routes by setting up blockades, creating a save dedicated to this platform ambush defence. Blocked enemies will probably stand around for a bit, wanting to go where they can't; but once you move back to an acceptable position, they should start getting drawn back along the inner platform again. If you don't have enough material to block off a route entirely, you could at least block some of it so the enemy would have to go further to get around.