Top pass retreat
Starting ideally with a far area horde set-up so you've got the entire covie contingent in the far area or not far off, drive in and go through the top pass (you could ditch the hog beforehand if you want). Your job now is to defend from near the gap (on that side, principally) against the mass of covies streaming up after you. Most of them will try to come through the pass, but you're also likely to get some coming around the rock to flank you, so watch out.
I think of this principally as solo work, but you can take Warthog buddies in if you like. The downside of having Marines along though, is that you'll probably get more flanking enemies than normal, making for a more scattered battle, harder to control.
Weapon considerations
The strongest weapon combo for this plan is surely the plasma pistol and plasma rifle. As an Elite or Jackal comes through or hangs around on the other side, a plasma ball can take down his shield, and you can finish him off with the plasma rifle or tag him with a plasma grenade while he's still stunned.
The plasma pistol is also superb for taking down enemies with rapid-fire bursts though. On Heroic at least, it's possible to fight the entire battle using solely that technique (using no grenades or melee), and with charge left to spare. The plasma pistol is very economical when used that way. Try it as a challenge. Perhaps see if you can even make it through with perfect health.
An assault rifle is excellent here too. It's very satisfying to drill covies at close range, but you might like to also have a plasma pistol for taking down Jackal shields. For a good challenge on Heroic though, try prevailing using only the AR and melee.
What about the needler? Sending streams of needles through the pass can offer some devastating results, but when Elites are so close with those nasty plasma rifles it can be hard to get off an explosive stream before taking damage. It's also quite slow to use, which isn't ideal here where you can so quickly overrun. So it needs to be used carefully, is what I'd say.
As for the pistol, it's not what you want to be holding when faced with an Elite at close range, but it's fine on the other covies, and you can have fun peppering them when they're still on the other side of the pass, perhaps standing around.
Grenade fun
You can have a lot of fun with grenades here. Covies on the other side of the pass make a great target and you can lob grenades though at opportune moments when they're approaching or clustering (sometimes hesitant about coming through, as if they're not quite sure where you've gone). There are good possibilities for some huge chain reactions there, especially if you've already killed a few Grunts or Elites. You can also bounce grenades off the cliff wall, particularly frags.
It's easy to tag enemies too, either when they're standing around wondering what to do, or when they've decided to come on through the gap. That's especially handy with Elites of course.
Watch that flank!
As mentioned already, you'll need to be alert to enemies coming around the huge rock to flank you (see pic). Sometimes it's just some grunty fool you can dispose of in a flash, but sometimes it's an Elite; so keep an eye on that motion tracker! From the speed of the dot you can tell if it's an Elite coming. If it is, you may want to lob a plasma grenade through the pass to dissuade enemies from coming though for the next few seconds while you're occupied. That delayed fuse is mighty useful sometimes!
Getting pushed back
If you start having to back off from the pressure, it's not uncommon for two Elites to come though the pass in close formation, and if they're both reds with plasma rifles you'll really be up against it. Tagging an Elite at close range while he's dodging about is a skill that will come in mighty handy in situations like that. Another important tactic is to close in so he's tempted into taking a swipe at you. You can then dodge while continuing to dish out punishment.
When more than one enemy is coming through at once and you're forced back, try to position yourself so the front guy shields you from the ones behind. On a good day he gets it in the back from his buddies, which is always good for a laugh.
Polishing off Grunts and Jackals
If I've killed off the Elites, sometimes I'll deliberately stand well back from the pass and enjoy the challenge of handling the lesser enemies coming at me from around both sides of the rock. Grunts and Jackals can get seriously aggressive when they think they're getting the upper hand, and it can be a lot of fun (also quite amusing sometimes when it seems like there's no end to them).
Impeding or blocking with the hog
If you like, your Warthog can be used to impede the pursuing covies, to slow the rate at which they're likely to come through the pass. That could let you tackle them more easily. You'll need to experiment with what works, in terms of positioning the hog.
More extreme, there's also the idea of simply blocking the covies. In that case you can end up with loads of them on the other side of the hog, at which point you can enjoy some leisurely mischief.
In particular, you could kill some Elites or Grunts to get some loose plasmas on the ground, then add a grenade of your own (or use an explosive stream of needles) to set off a nice big chain reaction, which as a bonus may also send the hog skywards! I wouldn't call this a proper top pass defence really, but it makes an enjoyable spot of fun at least.
Grenades and melee challenge
Here's a challenge you might like to try. Fight the whole battle using nothing but grenades and melee attacking. On Heroic it's tough but manageable, and relies on tagging a lot of Elites as they try to come through the pass. Often they spill more grenades, keeping you nicely supplied.