Top pass retreat
Setting up
Start 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. With a mass of covies streaming up the hillside after you, your job is to defend from close to the gap. Most of the enemy will try to come through but you're also likely to get some coming around the rock to flank you, so watch out. I normally tackle this battle solo but you can take Warthog buddies in if you like. The solo battling can be hugely enjoyable and is definitely something you ought to try.
You could also use a modified far area horde set-up, but because of the enemy's initial deployment (now partly spread out back towards the lifeboat) you're likely to get more of them coming up on your left, rather than trying to come through the pass. Still plenty of fun, but it weakens the intended nature of the battle somewhat.
Weapons and tactics
Grenade fun
You'll have a heck of 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). Good possibilities for some huge chain reactions there, especially if you've already killed a few Grunts. I look back fondly on the time I blew a couple of Elites halfway up the cliffside. Ah, happy days…
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. You can wait until one's coming, then step into view to stun and tag him (nasty shock eh, squid-face?), then quickly back off and wait for the bang.
Plasma recommended
The ideal weapon combo is surely the plasma pistol and plasma rifle. As an Elite or Jackal comes through or hangs around on the other side, the plasma pistol will 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. Tagging has the advantage that it turns the unfortunate enemy into a walking bomb, which in the case of Jackals and Grunts will usually head back among its fellow scum. Excellent!
The plasma pistol is also superb for taking down enemies with rapid-fire bursts. On Heroic at least, it's possible to fight the entire battle using solely that technique (no grenades or melee attacking), and with charge left to spare. The plasma pistol is very economical when used that way. Try it as a challenge; see if you can even make it through with perfect health.
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.
Pistol and assault rifle
With one set-up I've got, I'm armed with a pistol and assault rifle. That's not ideal here, but all is not lost. The pistol is good for taking down Grunts and Jackals before they get through, and for peppering Elites to dissuade them from trying. If an Elite does push on though, he'll either get drilled with my assault rifle or find a plasma grenade stuck to his chest; all of which should ruin his day. However, on Heroic at least, things can be pretty tricky if your only close-range weapon is an assault rifle, which - let's remember - is only mildly more effective than shouting "bang-bang-bang" loudly. Or at least, that's what it feels like sometimes.
Watch that flank!
You'll need to be alert to enemies coming around the rock to flank you. Sometimes it's just some grunty little fool that you can dispose of in the twinkling of an eye, but often 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 moments whilst you're occupied. That delayed fuse is mighty useful sometimes!
If you brought any Warthog buddies along, you'll probably get more flanking enemies than normal, making for a more scattered battle. For this reason I prefer to fight this battle solo.
Polishing off Grunts and Jackals
If I've killed off the Elites, sometimes I'll deliberately stand well back from through 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). Actually, if you fancy this sort of challenge, you can make things ideal by killing off all the Elites before your entry checkpoint, giving you a 'Grunts and Jackals only' set-up (perhaps at a higher difficulty level than usual, to increase the challenge). Of course, you could use such a set-up for any battle plan, not just this one.
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. Fatal error, Mr Squid-chops! His swipe is your cue to cooly backstep or sidestep while continuing to dish out punishment. And it's easy to step back in to whack him once he's missed.
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.
If you start to get overwhelmed and your weapon is overheated or needs reloading, a quick way to stop a nearby enemy shooting at you (which they tend to do all the more aggressively when they've got you on the ropes) is to tag him with a plasma grenade. Once he's got that thing fizzing away on his hide, that's all he's going to be thinking about. This has been a life saver in many a sticky situation when down to my last bar of health.
Run away!
If you've been pushed through the pass and your shield is bleeping, and your health is in tatters and there's a pack of nasty red Elites about to come through, and basically your whole cyborg world is collapsing around you, then you can always go grunty and "Run away!". It may not be terribly dignified but it's better than becoming toast. Retreating deeper into the rockslide, you can switch to 'hit and run' tactics, moving around in the rocks trying to take out enemies a few at a time.
Note: Sadly, research confirms that actually shouting "Run away!" as you do so does not increase your speed. Extensive testing also suggests that shouting "Stop shooting me!" in a pathetic little high-pitched voice has approximately the same effect on the Covenant as it does on Master Chief, i.e. none whatsoever.
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.
Blocking the pass
Sometimes I'll leap out of the hog just before reaching the pass. It slides to a halt and can optionally act as a partial or total blockage to the enemy that you can exploit as desired. When it completely blocks them, you can get some fun situations in which loads of frustrated enemy are congregating on the other side of the hog. In due course you can add grenades to the mix or send in explosive streams of needles. You may like to kill a few Elites or Grunts with weapon fire first, so the ground is littered with plasma grenades for a nice big chain reaction.