Plateau back defence
Head for the Marine plateau and defend from the back. This is especially enjoyable when you've got plenty of Marine survivors there because it's a lot of fun fighting alongside them. They retreat there when the final dropship comes in. I wouldn't generally like to have the sniper around, as I'm not keen on getting shot in the back and he's also a bit of a kill stealer, but even if you eliminated him in your set-up work, that potentially still leaves six Marines, making a squad of eight if you add a couple of Warthog buddies.
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.
You could even add to the jostle by arranging a few extra Marines if you like. Adding four would be pretty easy, giving you a massive squad of twelve! With that many, you might even consider just watching the Marines and covies fight it out for a while, though you may like to offer the occasional bit of help.
Trickle or swarm
Sometimes the covies 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. Either way, there's a distinct element of suspense as you listen out for the sounds of approach and watch for enemy heads coming into view over the plateau's gentle rise.
In the case of heavy attack, there can be a considerable danger of getting overrun, and at times like that you need to be mindful of the advanced fighting technique of running away. Sometimes there's just going to be nothing you can do to prevent your Marines being wiped out 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 may also need a decent amount of health left to survive a drop if you elect to go off the edge. However, crossing over to the island plateau is another possibility.
Warthog tips
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!
Also, definitely do not 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.
One other tip relating to the hog. This is just a bit of fun you can have if things aren't too hectic. After parking the hog 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 off, and peer over to watch it hopefully squash a few of the alien nasties. It takes a bit of luck, but it's worth it.
Grenade hoovering
If there are ever any grenades lying amongst the Marines, consider hoovering them up to prevent catastrophic chain reactions. 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.
Raging Marines
Sometimes a Marine will be on death's door and will 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 the Marine isn't killed, he can stay actively aggressive for quite a while, hunting down his foe and plowing shot after shot into alien hide.