Path top defence
Associated movies
Get to the vicinity of the top of the spiral path before any (or before many) covies have advanced that far - assuming that's possible with your save - then defend from there against the vicious onslaught that will soon be coming your way, and mop up the remnants afterwards. Getting into position fast enough may require flying there in a Banshee, but it depends how you've set things up.
Tactics
This is likely to be a very hectic and intense battle, but the rise in the path can give you some cover. Back off to recharge when needed. The Marines should come up the path to join you pretty quickly, and most of the enemy's attention will go on them when they're to the fore. There's an Elite and some Grunts under the nearby tree, and it's worth taking them out fast, particularly the Elite. They can otherwise be a near-constant irritant, and the Marines have a disastrous and annoying tendency to wander into the range of the Elite's plasma rifle or needler.
I recommend throwing a lot of grenades, particularly along the cliff edge where most of the enemy will be distributed. You can get some great chain reactions when loose grenades litter the place.
As for weapons, one nice combo is a plasma rifle coupled with a pistol. But I also enjoy slamming rockets along into the masses of enemy, and rockets are also very useful for taking out Hunters at distance, who'll otherwise tend to bombard you. A needler works very nicely, and if you keep well back you can have a good time with a sniper rifle too.
One thing you have to beware of is Elites eventually coming up the path behind you, after getting blown off the cliff or leaping off or stumbling off. If you suddenly notice a stream of plasma or needles hitting your back, that'll be them! When you see an Elite go off the cliff still alive, watch your back. More to the point, watch your Marines' backs, because an attack from the rear will really spoil their day.
Using a pass blockade
See also my pass blockade rumble plan. The 'early release' version of that plan (seen in BCM68) starts out as a path top defence initiated in a special way, namely by blasting a blockade to release the main enemy horde. It's a nice way of setting things up, because with the enemy held back, you're guaranteed plenty of time to get into position. To begin with, the only enemies free to come after you would be the dropship troops; but you can be ready for them before they're even off the ship.
Using a 6-Ghost blockade on the cliff
Another blockade you could use is line of six Ghosts across the wide cliff path, just near enough to the spiral path to hold back enemies from the dropship. You could potentially let enemies gather behind it before starting your path top defence, and you can even wait until the Marines have joined you. I'm glad I tried such a blockade because it turned out to be a whole lot of pyrotechnic fun. One enjoyable way of blasting it is to send in a couple of grenades to blast it, then send in a few rockets skidding low across the ground (from a position of cover slightly down the rise of the path). They're almost certain to hit the wall of enemy still remaining, taking out whole handfuls at a stroke. But more enemy will often remain, and for a while it can be complete bedlam as you fight for your life against the now somewhat annoyed enemy. Fantastic stuff. If you really want to sock it to the Covenant close-up in the most explosive manner, give this a go! You may like to eliminate the Hunters in advance however, else their blasts can disrupt the blockade early on.
As a bonus, this blockade can also be used as a good way of starting off a defence at the base of the spiral path or further back towards the underpass. Let enemies collect, then release them with a Banshee bolt or whatever, and get to your defence spot. You'll soon have a nasty released horde to deal with.
Note: When I first tried such a blockade, an Elite coming up the spiral path ran straight to one of the Ghosts and hopped in, ruining my work. On a second try at setting things up, I overcame the problem by quickly killing the Elite as I flew into the area (triggering him and other enemies). If you follow my advice on Ghost safety however, you shouldn't have any such problem.