Left side defence
Associated movies
The left side of the battlefield is my favourite to defend in, as it's nice and brightly lit. You can see your targets easily and the battle area becomes quite a spectacle as the enemy flood along. There are lots of possibilities and I'll talk about a few of them here.
Trees at back left
My favourite defence spot is back past the door, near the trees. The enemy have a long way to come, giving you plenty of time to work on them. Course, there are a whole load of 'em, so things can soon get rather hectic, depending on what weaponry you're using. You're guaranteed some marvellous fun with a sniper rifle, which would make a great combo with a pistol. A rocket launcher is great for slamming into clusters of enemy as they come around the distant corner of the pipe block. Get your timing right, and BLAM! - you can decimate them. Pretty hilarious. Long-range grenades are good for that too. See BCM89 for this sort of long-range work. You may like to kill some Elites and Grunts first to try to get some loose grenades scattered about, for a chain reaction.
You can potentially be joined by some released Marines (e.g. I've got a nice save where they're initially in a pen at the back, and I just blast it to let them run over), or you could actually have the Marines already cooped up in a pen based around the trees. That would be my favourite pen defence, and in BCM89 you can see a large squad cooped up like that, plus Sergeant Johnson on a chain-gun to the side. Yet another option is to have a long blockade, going from the cliff to the platform block.
When I've got the support of a large squad, my favourite combo is a needler and AR, such as seen in BCM17. It's not the most powerful combination of course; it's just a lot of fun, and makes things reasonably challenging (certainly far more challenging than using a sniper rifle). The barrage of fire from the squad tends to hold enemies up quite a bit, giving you time to send in explosive streams of needles or pepper them with the AR - a weapon which on Heroic would be dangerously weak in this situation if you were fighting solo.
With a load of Marines taking up much of the enemy's attention, I don't always stay near the trees. It can be fun to position yourself well off to the side, at the end of the pipe block. From there you can fire into the sides of enemies attacking the Marines. Jackal sides will be unprotected as they carry shields on the right. You'll have to watch out for quite a bit of fire coming your way, but you can use the corner of the block as cover. You could even jump up onto the block and use a pipe for cover.
Backing off
Another way I like tackling things is to land early, relatively close to the enemy, then slowly back off, taking down enemies with sniper rifle and pistol as I go. It can make a great challenge to see if you can wipe out the covies single-handed. In view of the high enemy numbers (assuming you've got them), you really need to be efficient with the sniper rifle to pull that off. Or at least, that's my experience on Heroic.
You could try this backing off plan with other weapon combos too, but on Heroic it would probably be pretty tough work in view of all the Elites galloping your way! You might need to thin out the covies first. For higher enemy numbers you might need to use Normal. See BCM327 for example, in which I use three different weapon combos against fifty covies.
Blockade fun
It's possible to stretch a blockade between the pipe block and the cliff, which can give some great battling. See Left side blockades for suggestions and construction advice.
In particular you can use seven Ghosts to form a well advanced diagonal blockade including a rock in the middle, as seen in BCM37. However, you'll probably find that the Marines spend an awful lot of time comically shooting each other, as you'll be able to see and hear in that movie. They don't have the sense to spread evenly along it, and instead tend to cluster near the middle with chaotic results. Mind you I did have a rather large squad in that save, which didn't help matters.
With that blockade you can defend quite nicely from on top of the huge cliffside rock where the blockade ends. Depending on the exact rock geometry (which gets randomized whenever you come across the loading point back in the tunnel), the rock may even afford you some cover. Even if it doesn't, I find that most of the enemy fire gets directed at the Marines, even though they fall back from the blockade when you're up there. It's a great spot (see pic). One of my favourite challenges is to try to take down most enemies before they even get near enough to trouble the Marines. See how well you can do, using a combination of the sniper rifle, rocket launcher and pistol.
For an alternative high vantage point, get up your Banshee's canopy - an idea I only hit on quite late, namely while working on BCM37. Clamber up a leg then jump on top. If you go to the sharp tip or sufficiently far forward on the nose, you'll no longer have the onscreen boarding prompt and will thus be able to manually reload. You're somewhat exposed on the canopy of course, but you get a pretty nice view of things, which is also good for watching the Marines in action. Try different positions for the Banshee.
Yet another high vantage point is afforded by the front pipe going over the trough. Dropping yourself off there is fairly easy; I suggest you point the Banshee along the pipe as you land, so you're lined up. The covies tend to focus on the Marines, though you do tend to come under serious fire from enemies close to the rock below on the platform. Still, it doesn't take long to lob a few grenades down there; or clear the area with rockets.
If you back off somewhat from that diagonal blockade or go up on the cliffside rock, the Marines get triggered into falling back. You might like to do this on purpose if you think they're taking too much punishment, or if you think the blockade is about to be breached by an incoming grenade (Grunts are a major threat with their plasmas). You could fall back to a secondary defence from a safer distance. Some other Marine behaviour is that if the blockade is breached (and they haven't fallen back), they'll tend to charge through the gap, at which point everything is chaos. Good fun!
Up on the door arch
Another novel high place to defend from is on top of the door arch, though it takes a bit of a knack to drop yourself onto it from a Banshee. This position may be best left for a situation where you've got a blockade which will hold the enemy back for at least a while, otherwise they could close in too fast for you to handle.