Island plateau defence

Get onto the 'island plateau' adjacent to the Marine plateau, and defend from there. You can easily jump the Warthog across the gap, or simply jump across on foot (actually, you don't even need a jump; you can just run across). If you take a chain-gunner, you can set him up with a good line of fire covering one side of the plateau, as well as the plateau itself should any Covenant break through.

One type of battle dynamic is where the covies tend to head for the Marines left on the plateau (assuming there are any). You then have the challenge of trying to keep them alive.

A sniper rifle is the ideal weapon for that, ideally coupled with a pistol. It can be pretty hard to succeed though, especially if the covies end up attacking not only from the front route but also going around the back. You have to be alert to covies arriving from either side.

As well as being a lot of fun, this is some of the best sniping practice you'll get, as you have to be relatively quick to bring down enemies. You can also have fun lobbing grenades, surprising the enemy who are more occupied with the Marines.

Another type of battle is where the covies fixate on you, something you can probably encourage by standing near the edge to make yourself known (or at any rate, distracting them from the Marines opposite). They can end up congregating down below, making excellent targets for grenades as well as short-range fire. When Elites are hurt, they often shelter directly below, against the wall of the island plateau. And then if you're not under too much fire from other covies, you can peer down and tag them. Hee hee!

Scope out those shifty Elites

From the island, I suggest that you also take the time to use the sniper scope to simply watch Bungie's magnificent aliens attacking the Marines; particularly the Elites. If the Marines are all dead and an Elite comes over to the edge to start taking potshots at you, again use the sniper scope for a close-up look. One thing you might be interested in seeing close up is an Elite with a needler, shooting at you and reloading. With practice you can dodge the needles even whilst watching through a scope.

Having a squad on the island plateau

It's possible to ferry Marines onto the island to form a decently sized squad there. These could be extras from outside (that's what I'd recommend) or they could be the rockslide survivors themselves. You can have some excellent fun with this.

After getting a squad onto the island and driving off to later get a battle start checkpoint, a good thing to do is to slowly drive in around the right, trying to lead enemies up the steep right path to the plateau. As they close in on you, continue onto the plateau and hang around a while, maybe firing a few shots to get attention, then jump across to join your squad (on foot or in the hog if you want the chain-gun). Lots of covies should end up flocking onto the plateau and attacking you from the edge; that's the best fun. You can set off some great chain reactions after killing plenty of Elites and Grunts at the edge. Occasionally an Elite may get blown onto the island by a grenade, or may jump across in an effort to get clear of one.

Tip: When dropping Marines onto the island, try not to have them close to the edges else they could easily get killed before you've got your battle start checkpoint.

Access from the ground

Depending on the rock geometry, it can be possible to get up on the island plateau by jumping across from a nearby rock (rather than from the Marine plateau), though it can be difficult to get up on the rock itself. I'll leave you to find the way if you're interested.