Extra Marines

If you've got a suitable blockade to hold back the enemy, the horde boosting method makes it possible to bring in Marines from earlier in the level, giving up to eleven roughnecks down on the ice, ready for battle at the start of your save. Quite a novelty! Stop press: actually, I've later realized that if you get an extra few Marines and an extra hog near the start of the level, you'll potentially be able to get up to thirteen in the megabattle; but I'll leave that elaboration of the plan to you.

The basic idea is this. After triggering the enemy to head for the underpass and collect behind the blockade, you then go back to bring along some Marines using the Scorpion; Marines you left blocked off somewhere so they couldn't advance. Crucially, the enemy stay at the blockade, so as long as it's not too close to the pass, that leaves you with a clear route down to the ice without taking heavy fire, even though it means driving off the cliff or the edge of the spiral path. If there were further Marines to bring along, you can potentially ferry in two more in a similar way but using the Warthog. Afterwards you can get a battle start checkpoint. Here's a more detailed step-by-step plan if you want one.

Prepare blockades and more

In the spiral path area there are three things to arrange. Build a blockade away from the pass, for holding back the enemy. Place a Ghost near the underpass exit, for sealing off Marines later. You're also going to need Banshees or Ghosts near the underpass to get you back past the gathered enemy. A single vehicle will suffice if you're planning for at most four extra Marines, but you'll want two vehicles if planning for up to six extras. Banshees are preferable.

Build a 'Marine blockade' earlier in the level to hold Marines back until you want them. Build it in the tunnel that leads out from the cavern, at the corner just before where you meet some Elites and Grunts. I put mine at the bottom of the ramp up to where the snoozing Grunt will be. Four overturned Ghosts will do, but you could also use Shades and the Scorpion. Actually, the Scorpion plus one Ghost will suffice (maybe have the Scorpion at an angle to utilize its diagonal length); but later on when driving the Scorpion away, you'll need to be careful not to squash any Marines, as they tend to cluster very close to the blockade. Don't use the two Ghosts outside the top of the rising tunnel, as the game later moves those under the dropship. Also, if you're using a Ghost which covies are allowed to used, overturn it to make sure no Elite will hop into it (see here for advice). If you use any Shades from the cavern, Grunts will go running off to man them later, and you should preferably stop them; easy work with a sniper rifle.

Note: there's a reason why I put the Marine blockade so far forward (basically the furthest it could be). A short way back in the tunnel, there's a loading point, and I was worried that if the Marines stayed behind it they might end up frozen, or that the game might even remove them (the latter can certainly happen). Actually, I tried this and they didn't freeze, but something else happened. The Marines I eventually took into the spiral path area with the Scorpion exhibited no MC-homing behaviour there. After dismounting, they just stood still, which wasn't much good. I suspect that the loading point business was responsible.

If the Scorpion isn't already part of your Marine blockade, at least leave it close by. Place a Banshee by the door in the first snowfield, for immediate use.

Play through to the Marine blockade

After emerging into the first ground battle later, play through to the Marine blockade, doing the best job you can of keeping Marines healthy. You can minimize Marine damage in the first battle by using the Banshee to divert and destroy the enemy, and you can aid the three Marines in the crashed Pelican area by blitzing the enemy with the Banshee or with various other tactics. In fact, apart from the earliest Marines taking fire for a few seconds before you've diverted the enemy, you can avoid Marines taking any fire at all if you work thoughtfully. You can end up with Marines in perfect or near perfect health.

Have the Marines close to you as you come through the loading point just before the blockade, else they might freeze behind you.

Fly on through and do the usual work

Fly on through, with the enemy chasing up after you. Be off quickly to minimize any possibility of the Marines diverting the enemy into a fight. You're now back on track with the usual set-up work for a spiral path megabattle. Continue with that, up to the zigzag path trigger, and let enemies collect behind the blockade.

Bring in extra Marines with the Scorpion

Lead the spiral path Marines through the underpass and seal them off with the Ghost you left handy.

Use a Banshee or Ghost to get back past the enemy. Flying is best, but bear in mind that you ideally want to keep a Banshee for later, for getting a battle start checkpoint and potentially using it in battle. You can actually get over a Ghost blockade simply by driving at it in a Ghost. You don't even have to raise the nose (using the 'A' button); it seems to go over fine, without disturbing the blockade. However, the enemy is likely to suffer some friendly fire, possibly even killing a few. You could also get past with the active camouflage, but you'll surely want to save that for potential mischief later.

At the Marine blockade, pick up outriders on the Scorpion and ferry them to the spiral path area. To get them past the enemy you'll need to drive off the cliff or the edge of the spiral path, depending on where the blockade is. If you spin as you go off, it can stabilize you enough to land on your tracks (you may want to practice this beforehand). Even if you topple over, Marines tend to take little or no damage except if they're actually squashed. Be careful if you have to flip the Scorpion back over; lead the Marines away from it first.

If you're bringing in a sniper, make sure he's riding on the side away from the enemy, as you don't want him killing enemies yet.

Maybe bring in further Marines with a Warthog

If you left any Marines at the blockade, they'll potentially run after you as far as the pass (unless you managed to leave the Marine blockade intact to keep them blocked). You can ferry in as many as two, as follows. First get the new Marines sealed off safely with the others. Get back past the enemy a second time, and fetch the Warthog from earlier in the level. Pick up Marines and get down onto the ice you did with the Scorpion, spinning as you go off the edge. Try to minimize any chain-gunning.

If you didn't want to bring further Marines along, they'll potentially attack the enemy later if you get near enough for them to home in on you. That can actually be fun, because whilst the main Marines are coming up the spiral path, you can be with the other Marines attacking from the rear, having flown up to them.

Finish off

Now you've got all your Marines, finish off by getting a delayed checkpoint. Either delay the door checkpoint from the twin bridges area (but preferably kill any new enemies you trigger there, to minimize any extra strain on the game) or delay a tunnel checkpoint. Ok, all done. One extra-rowdy battle coming right up!