Underpass defence
Defend from back near the underpass, where the ice path ends. Once the flow of enemies has stopped, seek out and destroy the remnants.
Because the enemy has so far to come, they have longer to get strung out, which is liable to give the battle a less hectic pace than in a typical ice path defence for example. If you want to raise the intensity however, you could do it by arranging a blockade somewhere (for example a 4-Ghost blockade on the ice as described in the ice path defence plan), letting enemies collect there, then releasing them. Or you could do it by collecting a path mob as described shortly.
Weaponry
When covies appear around the corner, you'll have a fair bit of time to work on them if you're using suitably long-ranged weapons. Obviously the sniper rifle would be great, and likewise the pistol. Those make a powerful and enjoyable combo, and one thing I like to do is see if I can take care of things with that combo, with just the ammo I start with (both weapons fully stocked). A rocket launcher can be a heck of a lot of fun too, especially slamming rockets up towards the small rocks at the corner, where enemies often pause - which you can encourage by peppering them with pistol fire or whatever. So hey, why not have all three weapons handy? If Elites start to get close though, you might want to bring a plasma rifle into play.
It's good fun to lob grenades up towards those rocks, and I especially like to aim for the small gap on the right, which is good for tagging enemies coming through or approaching. But remember to allow for the fact that grenades bounce quite far on the ice.
Wrong way fellas!
Amusingly, when you kill Elites and the Grunts or Jackals panic, they run towards you rather than away. Not too bright these guys! You might risk letting the Marines take care of them if you're a bit busy with more urgent threats, but you can't really count on them. A pistol is the best thing to have for some quick mopping up.
Ammo rock
Back at the ammo supplies there's a rock which is usually low enough to jump on top of for a slightly raised position. I quite like using this as a platform from which to execute a long-range defence with a sniper rifle and pistol. You need to be a bit careful when throwing grenades though. If the rock is relatively high, the branches of the tree can be so close above your head that your grenade hits them and bounces down around your boots - which isn't much fun at all!
Collecting a path mob
Here's something you can do at the start to make things a bit different and raise the intensity of the attack. Fly up to the half of the Shade ledge furthest from the underpass and let enemies collect on the spiral path, eyeballing you whilst thinking evil Covenant thoughts. As long as you're far enough from the underpass, that's about as far as they'll go. At this point you could of course have some terrific fun with a rocket launcher and grenades, as described in my high position defence plan, but that's not what we're doing here. When you've got a good enough crowd (or when you want to give your Marines a break from the fire they'll probably be coming under), get back down to your defence position and defend against the concentrated mob that will now come rushing after you.