Plateau side defence
Park near the left entrance to the plateau, so the gunner has a good line of fire down that whole side of the plateau - the side with the short tunnel feature. As for yourself, here are some ideas.
On the plateau
Go up onto the plateau where you can set up a crossfire from the edge and be in a decent position to protect both the Warthog and the plateau Marines. It would be fun if those Marines helped out at the edge, but they don't seem to go near it. Incidentally, you see the slab of rock sloping down forming the tunnel roof? If you're ever down at ground level, you can get up that rock slab and then hop onto the plateau. It's a shortcut that could be useful if you're being pursued by Elites.
Stay right
Move to the right of the Warthog. You can cover the tunnel and surprise Covenant coming from the right. You can also move around the huge rock to the right, to surprise Covenant from behind, such as Elites who've taken shelter to recharge their shields.
On the island plateau
From the plateau, jump over to the island plateau. This is a bit risky as it makes it more difficult to keep the Marines alive if they get attacked, or to guard the Warthog from Grunts with grenades. You also tend to receive fire from more angles. But maybe you like a challenge. A pistol and/or sniper rifle would be ideal here.