Island plateau defence

You can easily jump the Warthog over to the island plateau, setting the gunner up with a good line of fire covering one side of the plateau (as well as the plateau itself, should any Covenant break through).

Distant support

If you choose to stay on the island, you can use the sniper rifle and/or pistol to try to help keep the surviving Marines alive as they face the incoming waves of Covenant. 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.

Crossfire

Rather than staying on the island with the Warthog, you can jump off quick and head back over to the main plateau where you can protect the Marines and set up a good crossfire. Covenant will tend to shelter under the island plateau to avoid the chain-gunner or recharge their shields; and you can shoot them in the back or use grenades.

Chain-gun massacre

When bullets from the chain-gunner hit ground, the thudding sounds and visual effects are superb. You get a good chance to see and feel this close up when the Warthog is on the island plateau and you're lurking where the enemy is being massacred. Check it out! You almost have to feel sorry for those Grunts as they get scythed down for the umpteenth time.

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.

Marines 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.