Plateau back defence
The basic idea
Head for the Marine plateau and defend from the back. Most of the following discussion will be for the case where you've got plenty of survivors left, because it's a lot of fun fighting alongside them. They retreat there when the final dropship comes in. On my main setting up page I gave advice on saving the Marines for the battle. I wouldn't like to have the sniper around, but even if you kill him off beforehand that still leaves 6, making a squad of 8 Marines once you drop a couple of Warthog buddies. You could even add to the jostle by arranging a few extra Marines if you like. Adding 4 would be pretty easy, giving you a massive squad of 12!
I've also already given advice on how to simultaneously preserve the non-sniper Marines and the initial covies; see Saving Marines and covies. That would be ideal for this battle plan, though it'll no longer be practical to bring in extras (because the initial covies would probably start fighting with the survivors below while you were ferrying in your extras).
Trickle or swarm
Sometimes the enemy may tend to trickle in bit by bit, in which case you might have fun with a pistol, picking off arriving Grunts and Jackals with headshots. Other times they may be more inclined to swarm in, and it's a case of all hands to the pumps. In this case there's a much greater danger of getting overrun. Either way, there's a distinct element of suspense as you listen out for the sounds of enemy approach and watch for enemy heads coming into view over the plateau's gentle rise.
If the enemy is taking a while to arrive, one delight you can have is to listen to the sounds of the approaching Grunts down around the sides of the plateau. The clip-clop footsteps, the snorting, the amusing things they say; if you love those grunty fellows, you'll really enjoy this. To help engineer a slow arrival, I recommend that before driving in, you should let the Covenant disembark from the final dropship and settle down a bit.
They're alive!
Fighting shoulder to shoulder with the Marines, Bungie's speech programming really shines here, quite apart from the wonderful voice acting itself. The variety of lines is terrific. The Marines seem alive, and you can really believe you're toughing it out with a bunch of roughnecks. Also, even after many hundreds of hours of play, I'mstill occasionally hearing phrases I swear I've never heard before, such as when Sergeant Johnson angrily warned that "The next one who dies gets double duty - and cleans my laundry!". It's brilliant work, and hopefully a lesson to other game developers.
Marines vs covies
Here's a tip. Set things up on Easy and simply sit back and enjoy watching the Marines and covies fight it out. For once, the Marines actually stand some chance of seeing off enemy attacks, though you may like to offer the occasional bit of help. You'll have plenty of time to marvel at Bungie's work and move around to follow the action. Highly entertaining, and a welcome change of pace.
Coming through!
When you drive in with the Warthog, take care not to run over any of your Marines in the process. You'd think they'd keep well clear of a moving vehicle, but no, they're absolute idiots and sometimes even dive into its path, making you wonder why you're bothering to save them at all. It's even more galling when you get the blame. Sheesh!
One other Warthog tip. Whatever you do, don't place a chain-gunner behind the surviving Marines. Not unless you want to see him blindly mow down half the squad over the next few minutes. Mind you, give him credit, he does apologize when he slaughters someone. "Sorry soldier!". What a considerate guy.
When Marines go barmy
Sometimes a Marine will be on death's door and launch a final mad charge at the enemy. "Show me what you got!" is the sort of call you hear. Unfortunately, what the enemy have usually got is a stream of piping hot plasma. Still, you can have fun following up in support or just watching how well he does, if there are no Elites around. I've seen Sergeant Johnson gun down a cowardly retreating Jackal this way. Nice one Sarge! If he's not killed, the Marine can stay actively aggressive for quite a while, hunting down his foe and plowing shot after shot into alien hide.
Time to bail
If Elites have arrived in numbers, your Marines can easily start to get overrun, especially if you're cowering behind a rock waiting for your shield to recharge. Worse, when the enemy starts to overrun you, they tend to increase in aggression, with Jackals and even Grunts going into a murderous rapid-fire mode. So, the situation can very quickly go from bad to worse.
At times like that you need to be mindful of the advanced fighting technique of running away. Seriously! Sometimes nothing you can do is going to be able to stop all your Marines being dead in the next ten seconds or so. If you fail to recognize that situation, you'll be needlessly joining them, and there goes humanity's last hope against the Covenant! The window of escape is only short, and you're also going to need a decent amount of health left to survive the drop if you have to jump off the edge. So you need to constantly judge whether it's time to bail. They might not do it in the movies, but this is reality! Stop press: my psychiatrist says that actually it's not reality; but I reckon he's just messing with my head…
A few tactics
Keep that Elite dancing
When an Elite has pushed forward, he'll often take cover for a while behind the tree at back left. You can lob a grenade over to give him something to think about. On occasion they end up blasted clean onto the island plateau, still standing. Another tactic you can use is to place your Warthog buddies where they can put any such hiding Elite under fire, to make it harder for him to recharge his shield.
Grenade hoovering
If there are ever any grenades lying amongst the Marines, consider hoovering them up to prevent chain reaction blasts. You may have to throw away a few grenades first so you can pick up the new ones, but it's probably worth it. You can have fun lobbing grenades into the unseen distance, based on your motion tracker. It's great to hear the enemy reaction before and after the blast, with such blind throws.
Drop a Warthog on their heads
Here's another bit of fun you can have if things aren't too hectic. After parking the Warthog near the plateau edge, make the occupants dismount, then bash the hog until it's partly over the edge. When covies are below, whack it right off (or grenade it) and peer over to watch it squash a few of the alien nasties. It takes a bit of luck, but it's worth it.
Sniper rifle fun
If you've got a sniper rifle up on the plateau and the final dropship is only just leaving, you may have time to snipe a few recently disembarked Jackals in the distance. Ha! That'll teach 'em to skulk around like that.