More set-up tips
These ideas apply not just to the basic plan but also setting up with Marines.
In the basic plan I mentioned how you might like to assemble a stash of spare weapons near the underpass after you've got the covies through. But there are other ways of providing spares, both inside and outside the battlefield, leading to greater flexibility. In particular it's nice to have spares in the battlefield so you can swap weapons, either to alter tactics (possibly switching from long-range weapons to short-range ones as the enemy closes) or just because you're running low on ammo. Here's some advice on all this.
Early spares and place-holders
I often do some spares provision work in the 'rearranging' phase when most of the level is still deserted. You could do this as part of a base save, enabling you to reap the benefits in each battle you then set up.
Having done a descent trick off the first bridge, return in a Banshee, kill any remaining enemies there and throw weapons off the edge. These can be any that were dropped by enemies on the bridge or earlier in the level. Now fly down and move the weapons to wherever you want them, inside or outside the battlefield. It's a long way to go but you can move multiple weapons at once to save time; see my general advice on weapon moving.
Get needlers full on ammo before throwing them off. Plasma pistols on 100% charge can be obtained from Grunts on Shades, so make a point of letting Grunts man Shades before killing them. On Heroic and Legendary there's a plasma rifle on 100% from the Banshee pilot.
You'll doubtless want some UNSC spares too, but unless you care to kill some of the Marines standing around, all you've got at the moment are your original AR and pistol, neither of which will be full on ammo. But here's what you can do. Leave extra Covenant weapons as place-holders in the battlefield or wherever. Later when you get your hands on the UNSC weapons you want, you can fly in and swap them for the place-holders.
Reminder: whenever you encounter a weapon that you might want later, briefly pick it up to try to prevent the game removing it when you're not looking.
Spares on the underpass roof
In some of my saves I've got some spares on the underpass roof. The idea there was to eventually get a battle start checkpoint when hovering just back from the roof, and from there I could either fly straight into battle or make a quick landing on the roof for a weapon swap. In retrospect, it's questionable whether the arrangement is worth the effort when it's so much easier to just have spares on the ground near the underpass, such as seen at the start of BCM36 where I'm standing near my spares and Banshee, but here's how to get weapons onto the roof if you're interested.
For one thing, if you kill a high Shade gunner non-explosively later, you'll have a plasma pistol up at the right level (on the roof itself or the adjacent ledge), which you can always replace with a desired spare by swapping. As such, the three high Shade gunners give you a way of getting up to three spares on the roof. However, I'm not inclined to use the gunners for this as I'd prefer to get them into the battle.
Fortunately it's possible to blast weapons up early on with the Scorpion (the Banshee isn't quite powerful enough), and it's actually not that hard. I suggest you work as follows.
Put a weapon somewhere on the spiral path (see pic) and line up the Scorpion ready to blast it. Fly off to trigger a tunnel checkpoint, but keep it delayed with Banshee fire until you're back at the tank. Get aboard and blast the weapon. Try to keep sight of where it goes (maybe hop out quick and switch to a zoomed pistol view). Check whether it settled on the roof or ledge. If it did, you can now fine-tune its position. If not, revert for another try; and so on. Depending on how confident you are, it may be worth trying to get further weapons up before consolidating with another delayed checkpoint.
Rocket launchers and sniper rifles
In my base saves I'll usually have three rocket launchers on hand, ready to blast the hell out of those naughty covies. If you're going to want that many, make sure you've got one with you as you trigger the tunnel checkpoint after wiping out the enemy in the two-Wraith area. Any launchers that you're not carrying are liable to blink out of existence at that instant, even if they've been picked up at some point, and even if they're in plain view as you trigger the checkpoint. Not fair! There's something quirky about that checkpoint, and I've seen other weapons vanish too. I've never managed to get both launchers past it - let me know if you find a way - but it's possible to get both of the early sniper rifles past. One way is to simply keep Johnson alive so he'll carry it over. As with launchers, you can have three in all.
Other provision methods (not needing a descent trick)
Here are four other methods of getting spares into the battlefield, none of which need a bridge descent (if that was a problem for you). They're listed in order of when they're performed.
(1) Tunnel method
After flying up the rising tunnel, you can bring weapons up the tunnel as follows (having first moved them as far forward as you can without being detected). Throughout this procedure, make sure to avoid triggering the tower enemies yet. You'll need to stay high enough when near the tower.
First lead your tunnel pursuers far away from the exit; preferably to the very end of the pass into the spiral path area. Then fly back and spend a while moving weapons up the tunnel. You could do this on foot or using Banshee blasts. Before the pursuers get too close, fly back up the tunnel, doing your best to avoid fire. You've now made some progress. Repeat as necessary. Once you've got weapons sufficiently near the exit (it can be done in just two movement phases), you'll be able to blast them along from a safe height until well clear of the pass. At that point the pressure is off and you can move them freely.
On higher difficulties especially, you may be concerned about how much fire the Banshee is taking each time you make your escape out of the tunnel, though there are two fresh ones you could always take advantage of. You can vary the plan as follows. Instead of flying back down the tunnel, transfer to a Ghost and drive down in that instead, switching back to the Banshee when you re-emerge later. Done carefully, this can keep your Banshee in better health. However, you'll want to be doing the transfers well clear of the dropship troops who hang around the tunnel exit. I suggest using the Ghost which is to the left when you first exit the tunnel, but blast it onto the nearby ice patch first, or thereabouts. You can step anywhere there without fear of triggering tower enemies, and you'll also be a good way from the dropship troops.
(2) Blockade method
Prior to triggering tower enemies, build a blockade near the start of the ice path in the spiral path area. Four Ghosts will do, and you have the option of blasting them to the spiral path area from a safe height if needed. When you later trigger enemies in the twin bridges area, enemies in the spiral path area will try to advance to the underpass but they'll be stopped by the blockade. Once they're out of the way there, you can safely move weapons along from earlier and throw them down off the edge of the spiral path towards the underpass. They can then be moved into the battlefield, though you may want to kill some enemies there first. The blocked mob can later be released, though you'd want to be careful not to cause much damage in the process.
(3) Killing method
Prior to tempting enemies through the underpass, kill some of the ones triggered in the twin bridges area. The Ghost pilot and Banshee pilot at the back are two good candidates, as you'd probably be wanting to eliminate them anyway. Each dropped weapon can be moved to where you want a spare, and you also have the option of swapping it for some other weapon, such as a UNSC one.
If you want to make sure no enemies come through the underpass yet (potentially causing you trouble), block it with a Ghost.
(4) Late insertion method
You can even get weapons into the battlefield after it's teeming with enemies. Move a weapon to the top of the zigzag path (without causing the enemy to open fire, else they might damage themselves), then use a Banshee to blast it along the battlefield, staying high enough to avoid enemy fire. Try to keep it on the snow so you can see it easily. When it's near to where you want it, you can fine-tune its position by first drawing the enemy mob back to the start of the battlefield, then flying to it and briefly dismounting to move it. But prior to the whole operation for each weapon, I suggest getting a delayed tunnel checkpoint so you can retry if anything goes wrong.
Weapon spacing tip
As a general tip, don't put your spares too close together, especially in the battlefield. Space them out well enough that you can pick up any particular weapon fairly easily. In battle you may need to swap in a hurry, and there's nothing more annoying that picking up the wrong nearby weapon by accident!
Getting variations of a save
You can easily tweak a finished save to get a variation that plays quite differently, complete with a convenient new starting checkpoint; namely a delayed tunnel checkpoint, or door checkpoint if you haven't already had it. You could just play this variation for a while (leaving the original save intact), or you could actually save the checkpoint to make it permanent. It's an easy way of broadening your battle experiences. Here are the main adjustments I'm thinking of. You could combine multiple adjustments before getting your checkpoint.
Adjusting the enemy layout
In a standard enemy layout, most of the covies are located at the start of the battlefield, concentrated around a few trees and rocks. When you fly into battle, the enemy attack dynamics will depend partly on how they were arranged - though it'll also be affected by how you fly about before landing. You can adjust the enemy layout as follows, in the hope of getting noticeably different attack dynamics which might be better (e.g. maybe the enemy will tend to come at you in one relatively constant stream, rather than two distinct attacks with a lull in between).
Trigger your checkpoint and keep it delayed. Initially you'd use Banshee fire for that, but once you're in the battlefield it can mostly be delayed just by enemy threat as long as you're flying low enough; e.g. around the height of the first ledge. Draw free-roaming enemies away from the start (there are lots of options there). Some may initially stay put, but if you fly back to get their attention they should get moving. When you've herded the enemy as much as you want, head back out and get the checkpoint - possibly after waiting a while if you want to give the enemy more time to flock back to the start.
In the case where they have enough time to return to the start, you've basically done a reshuffle, getting a new standard layout. If you give them less time, some of them will be further up the battlefield than normal. In that case you can expect to get attacked more promptly, and the pressure may well be greater. In an extreme case, a large mass of enemies may be far away from the start, and you could end up getting attacked from an unusual direction.
There's also a way of making the enemy move up the battlefield without even flying in. Land on the ledge next to the underpass roof, or hover low over the ledge, and you'll see the enemy immediately starting to shift. They seem to go about as far as the Shade platform, settling in a thick line across the field - and you can always draw them back again by moving to the left side of the roof. Amusing stuff. I sometimes fly over that ledge when heading into battle, to make the enemy start advancing early.
Starting inside the battlefield
Rather than starting from outside the battlefield as usual, you can start from inside, either on the ground - perhaps standing by some spare weapons ready for selection - or in the air. With a delayed checkpoint in hand, just fly to a place where you're sufficiently far from all enemies that the game will give you the checkpoint. You could just ascend high, or you could fly to a suitably remote defence position and get the checkpoint shortly after dismounting. With a checkpoint on the ground, you'd have minimal delay until the fighting. Nice!
Prior to getting your checkpoint, you might want to adjust the enemy layout somewhat, depending on the attack dynamics you're looking for. You could potentially even draw everyone to the back of the area then fly to the start to defend from there.
A typical aspect of battle is that lots of fast-moving Elites arrive first (possibly with one or more chasing Hunters, if you've allowed them into the battle), with Grunts and Jackals trailing behind. It's particularly noticeable when defending deep, as there's more time for Elites to outstrip the slower movers. You could potentially alter that dynamic by arranging for the slower movers to start off closer to the defence position. Here's the idea. Get all enemies in a bunch, well towards your planned defence position, then quickly fly to the start of the battlefield. After a few seconds the enemy should become drawn out, with lots of Elites at the head of the pursuit, hence furthest from your defence position. You're now ready to get your delayed checkpoint - and you might even be able to get it at the defence position. The slower movers should now have a head start in the attack charge. In practice things may not work out quite as well as I've painted it, but I think it's worth trying.
Eliminating some enemies
You can do some enemy selection by whittling out certain enemies, typically by landing on the underpass roof (possibly even the pinnacle) and sniping. For example you might fancy getting rid of the Hunters so there's not such a need to take a rocket launcher into battle, or maybe you want to weed out a few Elites to make things more manageable, or maybe you fancy the novelty of just battling against Grunts and Jackals. Selecting your personnel like this can allow you to use tactics that might not normally be viable (e.g. seeing if you can survive with just a pistol and grenades). Once you've done the elimination work, you can get a new checkpoint if desired. You'd probably want to for replaying.
In some of my saves I've actually got a spare rifle already waiting up on the roof to make elimination and topping up with sniper ammo more convenient. But that sort of convenience won't matter much in cases where you're going to get a new checkpoint afterwards.
If you've got Marines, you may be able to make some adjustments to the squad, or adjust aspects of the defence arrangement. In particular you could adjust the position of a supporting hog by flying in, moving it, then flying back out quick before enemies get too close. If the hog is behind a blockade which is holding back some Marines on foot, you could also adjust whether or not the hog is manned, or you could get a different gunner for a change in the vocals.
Another type of adjustment is to actually eliminate some or all of your squad, e.g. with Banshee fire or distant sniping. You may be declared a traitor of course, but that won't matter if you're eliminating the whole squad to go solo. If you leave some alive, you'll potentially have covies and Marines after your blood. Nasty!
Adjusting or adding spare weapons
If you've got some spare weapons in the battlefield, you can fly in to move them using a couple of techniques. You can blast them along from up high, which is suitable for moving them a long way, or you can briefly land to move them on foot. With the latter, you won't have long because Elites will close in on you fast. So you may need to do the work in stages, each time flying off before they get too near, then drawing enemies back to the start before you return for some more weapon moving.
There's also the possibility of adding spares to the battlefield, using the late insertion method described in my section on spare weapons (blasting a weapon past the covies and beyond, from a safe height).
Adjusting rock geometry
You can adjust the size and shape of the rocks in the battlefield. Remember, rock geometry gets randomized each time you cross the loading point after the underground bridge. You'll necessarily be crossing it if coming back from triggering a tunnel checkpoint of course.
A change in rock geometry can make quite a difference in certain locations, and may make some new defence tactics possible. For example, a rock that was previously too large to shoot over may now be just right. Or, a huge rock that you like to get yourself perched on top of may now offer better cover due to how it's shaped on top. If you don't like the look of things when you return to the battlefield, there's nothing to stop you going back to get things randomized again. Simply keep re-crossing the loading point until you're happy.
When you fly into a typical battle, there's a mass of covies near the start. If you've thoughtfully left a Shade or Ghost in front of where they're going to be, you can potentially enjoy blasting it as you head in, to send it flying towards them. It can provide a bit of fun each time you arrive.
There's good comedy potential here. I get a kick out of making enemies dive clear, but you can be vicious if you prefer and try to get a few squashed. It's fun to hear the sound of enemies dying and Grunts panicking as you fly off. There's an example in BCM17.
A Shade seems the best choice as it gets airborne rather better than a Ghost and moves around more before coming to rest. You could use the one from the nearby platform. You'll probably want to overturn it in advance, else it'll get manned. However, if you like the idea of blasting a grunty gunner each time you fly in, by all means leave it upright! The gunner will run to man it even though it's been moved.