Ice path defence
Associated movies
Defend around the mid-section of the curving ice path until the enemy flow stops, then head up the spiral path to wipe out the remnants. This is my favourite defence plan, and I have great fun with it on both Heroic and Legendary.
Defending from the outside corner
Often I defend from the outside corner of the ice path. From there you get a great view of enemies as they come around from the bottom of the spiral path. As with the path base defence, it's practically a case of enemies queuing up for treatment. In my case that typically means more pistol fun, with a few rockets and grenades for good measure. For a tougher challenge I'll try to stick it with a pistol and a plasma weapon, or some other combo. The Marines (if any) will be mostly off to your right around the rocks, so for once you don't have to worry much about loose frag grenades being fumbled around your boots. Phew!
Mind you, there's no natural cover here. Plasma balls from distant Jackals are easily dodged, but things can get a bit hot if the Elites start to get well advanced. You may like to position a Shade down here in advance (doing some rearranging), to potentially use for cover.
Sometimes you can come under fire from an Elite or Jackal peering over the edge of the spiral path, and it can be quite an irritation! It's even worse when you're defending closer to the inside corner. After all, it's hard to concentrate on the main attack if you've got a red Elite pumping plasma into your side! A rocket slammed into the edge is a handy way of dealing with the problem if you can spare the time. Another idea is to use a homing plasma ball followed by pistol fire. Or try a nice stream of needles!
Bearing in mind all the Grunts and Elites you're killing along the ice ahead, you can get some great chain reaction blasts if you let the loose grenades accumulate before sending in a rocket or grenade. Talking of grenades though, I find it good fun to lob them right up to where the covies come into view, sometimes even anticipating their arrivals. With that tactic and long-range fire from the pistol or perhaps a plasma weapon, you can decimate the covie ranks before they advance far.
There's plenty of fun to be had with a sniper rifle too, and because you're so far back you can work without coming under too much fire, especially if you can manage to keep dropping enemies almost as soon as they come into view. There's a rock against the back cliff just nearby which is usually a handy size for jumping on top of for a slightly elevated view. I quite like using it as a sniping platform. See if you can deal with the whole attack using a sniper rifle and pistol.
Off in the distance you'll probably notice a few Jackals hanging around a rock up on the cliff, over on the far left. When I've got a spare moment, I just love to send a rocket slamming into the rock (when big enough to be seen), sending the Jackals flying off almost horizontally, arms flailing. Highly amusing. You can also try slamming rockets into the cliff to take out enemies filing along; quite a challenge of your timing as they're so far away.
Defending from the inside corner
On the inside corner of the ice path there are some rocks that can be just the right height for shooting over while giving you decent cover. It's well worth getting a save with low rocks like that, because a defence from behind them can give some superb fighting if the enemy floods along in a good stream. A plasma rifle is great here, along with perhaps a plasma pistol or assault rifle as backup. And shovel those grenades! With the enemy coming more or less straight towards you, you can also do good work with a needler here.
You'll tend to have Marines around you, which is great for atmosphere but they'll sometimes wander out in front of the rocks, getting in your firing line. Predictably enough, shouting "Fall back you idiots!" doesn't seem to help. You then have the challenge of trying to fend off the masses while simultaneously trying to miss your own guys. I have to admit, I sometimes get sufficiently annoyed to take a more robust approach, along the lines of "You wanna step into my stream of plasma, go right ahead bozo!". What can I say, I'm a bad cyborg!
Things can quickly turn bad when enemies start to get past your defence line. Be aware that it's a poor idea to leave Marines to take care of the occasional Grunt or Jackal who comes running though in a panic. Except for the sniper (who I'd usually do without in this battle plan), the Marines are chronically bad at finishing enemies off. You'd better try to find time to do the job yourself, when you're not too busy fending off nasty red Elites intent on filling you with plasma.
Grenades
One thing you'll soon notice is that grenades take a quite a bit longer to come to rest on the slippery ice, especially frags. They bounce further. You'll need to take this into account if you want grenades to go off in the right place at the right time. To be effective you have to throw shorter and earlier, and anticipate enemy movements better.
Up on the huge boulder
Another nice place to defend from is on top of the huge boulder back towards the ammo stash, on the right hand side as you go along the curving ice path towards the underpass. It's a great position for a bit of pistol marksmanship as the enemy round the corner and get personal with the Marines. The sniper rifle and rocket launcher are the other obvious choices; normally I'd use one of these with the pistol. Sometimes this rock is climbable, other times you'd need a Banshee to drop yourself off on top. If you're creating a save dedicated to this battle plan, I'd recommend making sure you get a rock which is climbable, as that's more convenient. See my advice about rock geometry.
Using a blockade
I often play things with 4-Ghost blockade across the ice, about a quarter of the way along the ice path as you head towards the underpass. It's a very easy blockade to build. I've had loads of fun with a set-up like that, and when the blockade gets blown to pieces it's fun to see the Ghosts go skidding around on the ice, occasionally crushing enemies. I usually blow the blockade fairly early when there are just a handful of covies gathered, but if you're fighting solo you could wait longer. If you've got Marines however, they'll probably throw frags pretty soon which is likely to disrupt the blockade, so there's no point waiting around.
Sometimes the Ghost go sky-high in a big chain reaction if there are loose grenades from killed covies - and possibly even from a killed sniper (I like to eliminate the sniper in advance). When it's raining Ghosts, watch out that one doesn't land on your head! Once the Ghosts are scattered about on the ice, you can enjoy rocketing or grenading them to take out nearby enemies with the blast, or trying to make them careen into enemies. Playing 'covie skittles', as it were. The scattered Ghosts can also impede the enemy advance, giving you more time to work on them and keep them back. Of course, there was nothing to stop you having additional items lying around beforehand, making things even more congested.
A blockade such as this will also be handy for stopping the Marines reaching the base of the spiral path and getting into an early skirmish with covies. As such, it makes it easier for you to set up a good battle, in which no hostilities have yet broken out.
Motion tracker
One last thing. In this battle plan your motion tracker comes in especially useful. The spiral path is just off to your right and you'll see those red dots winding down it. You can anticipate what's coming (maybe even judging what's an Elite by how fast the dot is moving), and react accordingly. In particular it can help you decide when you've got time to pick up new weaponry, or when to do reloads.