Plateau side defence
On one side of the Marine plateau there's an expanse of relatively high ground, which includes a tunnel feature formed by a thick sheet of rock overhead. The plan is to defend against the covies as they mostly swarm into this area.
Ideally you don't want any covies coming around behind you from the other side of the plateau. Or not many anyway (and especially nothing eight feet tall). So you'll want to do what you can to encourage the desired enemy arrival dynamic.
Thus far, I haven't actually explored this battle plan very extensively, and hopefully I'll fix that in due course and perhaps expand this page. But I think there's plenty of potential in it.
Chain-gun assistance
You can give a chain-gunner a nice line of fire down this whole side of the plateau if you want, and since things will probably get quite tough (that's how it strikes me anyway), that might be a good idea! But there are many ways you might configure things, so be sure to experiment.
Cover options
Since there's not a lot of cover in the plateau side area, you might like to try using your Warthog as cover, positioning it suitably. It could also act as a blockage in the way of the covies, either blocking off an avenue entirely (blocking off the tunnel would be easy at least) or helping to funnel them into a kill zone.
However, you do have the rock near the plateau entrance. That's probably going to feature quite a lot in your battling, especially if you get pushed back by enemy pressure. If things are bad enough you could even back off onto the plateau itself of course, but at that point your battle plan would be getting somewhat lost.
Crossfire idea
That said, if you have the hog lined up for raking the covies on their approach, one idea for your positioning is to 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 hog and the plateau Marines. That's still a case of slaughtering the covies as they head along that side area, even if you're not on that ground yourself. So I think it sort of fits in with the general theme.
Tunnel focus
One particular idea you might like to pursue is holding off covies coming up through the tunnel. They don't have much room to dodge, so they make excellent targets and things can be rather congested. Throwing a grenade into the tunnel is an obvious idea of course, and if a few loose plasmas have built up there from your earlier work, the result could be spectacular!
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.
Extra marines?
Although I haven't tried it yet, I think this battle plan might benefit well from having a bunch of extra Marines there. Adding Marines for a plateau front defence is viable, so hopefully it can be done for this area too.