Flood onslaught challenge
The basic idea
Shortly after the start of level 9 you encounter a drop down to the surface. Cortana initially suggests looking elsewhere, but then the Flood start dropping out of the ceiling and she quickly changes her mind. If you're fool enough to hang around, you face a seemingly never-ending onslaught designed to force you to jump down into the pool of coolant. However, if you can stick it for two to three minutes the attack will eventually end. So the challenge is to survive it, on your chosen difficulty level.
I've had a real blast with this and highly recommend it. Despite the cramped setting there's lots of scope for tactical thought here, making for an absorbing affair as you try to hone your Flood-bashing technique. Thanks go to fellow cyborg Doug Auclair, whose comment prompted me to see if the onslaught ever ended.
No cheating!
One condition though: you're not allowed to go back to within a few steps of the Flood spawning area. If you do that they seem to stop spawning, and only resume once you retreat back down the corridor. You could therefore use it as an artificial tactic for alleviating the pressure. But we're not going to stoop that low, right? We want the full non-stop onslaught here!
Stragglers
Flood that spawn around the corner seem to get attracted your way partly by your gunfire, but at the end you can sometimes be left with a few stragglers. To make sure you've dealt with everything, head up towards the corner and either fire a shot or take a look.
Setting up
Here's my suggested routine for setting things up for repeated play, featuring a starting checkpoint that comes just before the Flood begin spawning.
Get the first checkpoint
When you start the level, go straight ahead and let the cute little Grunt and his gurgling pursuer go past. Run grunty, run! Turn left and get the three plasma grenades near the active camouflage. You may as well use the camouflage too. You can get a checkpoint about now or as you head off to the Grunts. It was actually triggered as you reached the first door that opened for you, but as long as you didn't dawdle, it will have been delayed due to the proximity of the Grunt and Flood. Pause before advancing to make sure you get it, if you haven't already had it.
Kill all enemies
Let the Grunts and Flood fight things out - or maybe contribute if you want to speed things along and stop Grunts lobbing grenades about the place - then finish off the survivors and pick up a fourth plasma grenade. But don't go down the main corridor towards the door yet; I'll say why in a moment. The carrier Flood is often out of sight near the door, but if you fire a few shots to get his attention he'll come into view and you can make him pop.
There was one Flood with a shotgun and another with a plasma rifle. When you get the shotgun it seems to have between 18 and 30 shells. The shell count seems to get randomized sometime after the checkpoint you got. So if there aren't enough shells for your liking, you can always revert and try again. The plasma rifle's charge also varies, but it's often 100% which is nice. If you want the full whack, you may need to revert a few times until you get it.
Neutralize the hangar
When you go a few steps down the main corridor, you'll trigger a conflict in the hangar across the hole. Before you do that, preferably get armed with at least one Covenant weapon that you're not interested in saving for your later battling. I'd suggest having a full needler and spare plasma pistol (not the fully charged one you initially had, which is better saved for the onslaught challenge). Go down the corridor, turn right and go just far enough to cause the door to open; namely just past the start of the purple panelling. Be careful not to get halfway along this short section yet, else you'll trigger Cortana's speech and the Flood onslaught, which you don't want yet.
Kill the Grunt and two Elites across the hole, as well as any Flood that have jumped down to get them. It's usually quite easy with streams of needles and maybe a few plasma balls or long-range plasma fire, but sometimes an Elite goes out of view to the left and you might not be able to get him. In that case I suggest you revert for another go. Occasionally a remaining Flood can end up standing behind a case across the hole, but he can be grenaded with a plasma and usually you can go back to get topped up with another, so that's ok. The reason you want to kill these nearby covies and Flood is that otherwise their presence may shortly cause your intended save checkpoint to be delayed unacceptably, and also you may end up taking fire in the back when desperately fending off the Flood in the onslaught challenge. Not nice!
Prepare for battle
Go back and get armed with your desired starting combo. If you like, you can even move some spare weapons along to your defence area, enabling you to switch weapons before or during battle. Luxury!
Get a convenient battle start checkpoint
Now to get your battle start checkpoint. Move about halfway down the short section of corridor that leads to the door, to trigger Cortana's speech: "Analyzing damage. This hole was caused by some sort of explosive; very powerful if it tore through the ship's hull. All I detect down there are pools of coolant. We should continue our search somewhere else."
Flood are due to start spawning at the scheduled end of this speech, so you want to get your checkpoint just before that. You're due a checkpoint once you trigger the speech, but you can delay it for a few seconds by jumping, during which time you can move to your desired starting position with the spawning area in sight. When you stop jumping you should get the checkpoint (bearing in mind that because there's no fighting in the hangar and you're not under immediate threat, the checkpoint won't be delayed further).
When exactly should you stop jumping? If you want the checkpoint a second or so before the Flood start spawning, wait until Cortana has said "detect", or thereabouts. But if you collected some spare weapons and want to always have enough time to get ready with any particular combo, only wait until she's said "explosive", or thereabouts. That'll give you an extra few seconds. Incidentally, although Cortana hasn't finished her speech yet, she fortunately won't continue it each time you restart your battle.
Experiences
Here are some of my early experiences with the challenge. There are a few implicit sub-challenges here to do with imposing weaponry limitations for extra difficulty, if you fancy giving them a go. With any such challenge, you can moreover attempt to survive with perfect health.
Let's try Normal first
I tried it on Normal first and survived using a plasma pistol and shotgun. I was also able to get through with just a plasma pistol on 100% charge; no grenades needed. After that I managed to survive using just the AR and grenades, though it was quite a bit harder.
Moving up to Heroic
Next I switched to Heroic and was again able to survive, though it was a lot harder. It struck me that the spores moved much faster, which made them more troublesome. I used the plasma pistol a lot, plus grenades, and brought the shotgun into play when things got sticky close up. One challenge I eventually managed was to survive using just a plasma pistol (no grenades).
On Normal it was possible to hold off the Flood from close range up near the spawning area, standing about halfway up the corridor where there's a recess that can give you a bit of cover if you want it. But on Heroic the pressure was too great and I always had to retreat back to the far end.
Resisting the voice in your head
As I was finding, it's not just the Flood you have to resist. You also have to resist the increasingly dire warnings from your in-house AI dreamboat Cortana, who really really wants you to make a hasty exit. I mean, she does her best to sound respectful about it, but reading between the lines it all pretty much boils down to "Jump, you tin-plated fool!". Apparently the little sweetie has no confidence in you whatsoever! And after all we've been through together. Now that's just dispiriting…
And now Legendary
After a while I gritted my teeth and gave Legendary a go. I'm tempted to say "Don't make the same mistake!". It's so hard at first that you probably won't know whether to laugh or cry. I think it may even be tougher than that bit in the Library where you get trapped between two doors. But after a number of attempts and a good deal of perspiration, I managed it using a plasma pistol, shotgun and grenades. Eventually I managed to do it while retaining perfect health too. Yes, it's possible, but you'll need to be right on top of your tactics and fight like a cyborg possessed!
Later I played on Legendary a lot and found it becoming significantly easier as I honed my technique. Nowadays I can actually survive around 40% of the time, often with full health - such as shown in BCM3.
AR challenge
Finally I managed what turned out to be the trickiest challenge of all: surviving on Normal using only an assault rifle (no grenades - which makes all the difference). My central tactic there was to use quite a lot of staccato firing, as elaborated in the tactics section below. This battle is a great place to get some intensive practice with that technique, but do bear in mind that this challenge is extremely difficult. If you're finding it too tough, you could always make an easier challenge by allowing yourself a limited number of frag grenades - e.g. just your initial four - to help you in sticky situations.
You can see me survive the AR challenge with perfect health in BCM4. In that particular play an AR gets blown my way early on, topping up my ammo rather conveniently. It's not an uncommon event to get an ammo boost from a low-flying AR, but you can also deliberately let a Flood with an AR close on you, then kill him to get his ammo. All in all, you shouldn't have much trouble keeping stocked with ammo.
Tactics
Having trouble staying alive? Keep these tactics in mind, some of which you can see in action in BCM3.
Lower difficulty while learning
Don't let yourself in for too much frustration right away. If you're finding things too hard, try fighting at a lower difficulty level for a while. With the reduced pressure, you'll have more time to execute the tactics such as described here. When you're fluent with them and fighting in complete control, step up the difficulty again.
Plasma weapons are better than the AR
I don't know why people think the AR is the thing to use on the Flood. The plasma weapons are far superior. As if the shot damage isn't weak enough, the AR's reload time makes it even more of a liability in this sort of situation. To get a vivid demonstration of just how much more effective the plasma pistol is than the AR, try surviving on Normal using just a plasma pistol or just an AR (no grenades in either case). The latter (the AR challenge) is far harder.
As for comparing plasma weapons, the plasma pistol beats the plasma rifle by quite a way. It does more damage, lasts much longer, and seems less prone to overheating. It's a real powerhouse when used well. Don't bother with the overcharge feature here; rapid bursts of normal fire are far more effective.
Conserve your shotgun shells
If you've got a shotgun, try to save your shells for when you really need them. None of your attackers will have shotguns, so you're limited to the shells you start with (which can be as many as 30 if you're lucky; see my advice on setting up). Usually I only bring out the shotgun when I've been forced back by multiple enemies and it's time for some close-quarter stuff. Mind you, a plasma pistol is also pretty good for close-up stuff and I often fight my corner with a combination of these two weapons. When you've got two attackers close together it can be worth aiming in the middle to score hits on both. Even if it doesn't bring them down, it'll usually stun them.
Staccato firing from the AR
When using the AR make good use of staccato firing, in which rounds are fired separately by rapid twitches of the finger. This is far more economical than using bursts, yet has a comparably rapid effect once you get the hang of it. The most important benefit here is that it greatly reduces the frequency with which you have to reload. Reloading is agonizingly slow and leaves you in danger of getting overwhelmed if there's still a bunch of Flood coming up the corridor. If you use bursts all the time, you're more likely to run your magazine dry before you've wiped out the latest wave. Mind you, sometimes a burst will be needed to take down Flood quick before they lash you or whatever. But whatever you do, try to pick your reloading times intelligently.
Needler not ideal!
Much as I love the needler, it's hardly the ideal choice here. You might be able to risk the occasional use if you pick the right moment, but in general it's going to be too slow-working.
Keep the spores whittled down
It's important to keep the spores whittled down to avoid being continually nibbled at, and to allow enough time for shield recharging. Plasma weapons are ideal for this, especially when there's a mass of spores up the far end of the corridor. A few shots can cause a chain reaction of popping, though the effect becomes less pronounced the higher the difficulty level. The AR is good too of course, but try to fire individual shots to conserve ammo. On Legendary I find the plasma weapons easier for spore work. You can also use melee of course.
When you're sheltering near the door, spores tend to collect at the end of the recess along the corridor as if they were trying to get to you via a straight line. When you move out, you've got a good opportunity for popping lots at once via chain reaction, due to how bunched up they are. Careful you don't get surprised by such a mob.
If you can spare a grenade, particularly a frag grenade, that can be a good way of cleansing an area of spores. However, I wouldn't normally be doing that unless desperate to get my shield recharged. Grenades are more valuable against combat Flood. At times you may be under such pressure from combat Flood that there's no time to deal with spores, or not enough to deal with them completely. But at least keep moving around so the jumpy little devils keep missing you, otherwise they can be your undoing.
Grenade use
The Flood will provide a steady supply of grenades, so it makes sense to try and use as many of them as you can. Watch out for dropped grenades and try to pick them up - which may sometimes involve a quick dash up the corridor. Whenever you're topped up with a certain type of grenade I'd suggest you use one soon, though it's not as crucial on the lower difficulties. On the higher difficulties good grenade becomes more important.
Look for concentrations of Flood to grenade, for maximum payoff. You can often throw a grenade up to the far end to take out a load of them, some of whom may be just out of sight, approaching from around the corner (but you know they're there). I do that a lot, and after a good bang like that you've potentially got a few moments to cleanse the area of spores to give yourself a nice clean corridor again. It's usually also a good time to reload if needed.
In general you should try to avoid tagging Flood, else you can end up with the grenade coming right back to you, fast and fizzing! When Flood are close, aiming for the floor or wall is probably safest. Sometimes I'll pretty much drop one where I stand, then back off, letting my attackers run into the bang. If you ever do accidentally tag something coming your way fast, try to stop it quick and back off. The only time I might tag a flood deliberately is if it's a carrier form a good way off.
If you've been forced back towards the door, you could drop a grenade near the corner to stall further Flood and give you a chance to get back in control. Grenades can also be handy for buying you enough time to recharge your shield. Clean corridor and a healthy shield; strive to maintain these things.
Carrier exploitation
Carrier forms are walking bombs that you can exploit. Try to pop them when they'll do a lot of collateral damage among other Flood coming up the corridor. That can save you from delving into your stock of grenades or whatever. Normally you'd set them off with gunfire, but remember you can also induce them to pop just by getting up close to them. Occasionally I do that then beat a hasty retreat. Keep aware of loose grenades on the ground and don't get caught in a chain reaction!
Sometimes you can deliberately wait until combat Flood reach a carrier before you pop it. On Normal I had plenty of fun with that tactic but on higher difficulty levels it's harder to pick out carriers, partly because there tends to be more Flood in the way, and partly because you're just plain under a lot more pressure. Talking of carriers, watch out for any getting blown down to your end of the corridor, either by grenades or carrier pops. An unexpected nearby pop can really ruin your day.
Here's a tip for buying yourself some reload time (particularly with the AR). If you see a carrier form waddling up the corridor, run up to it, reloading as you go and using it as cover from Flood further back. Approach to make it prepare to pop, then retreat. The bang will hopefully damage Flood coming up behind the carrier - depending on how cleverly you timed your approach - and by now you're reloaded.
Cover and motion
Assuming you're defending from the end of the corridor (back near the door), make good use of the corners as cover. I use both sides but the door side obviously gives you more room. It has two other advantages as well. Firstly, you have your gun on your right so you don't need to move out so far to get a line of fire. Secondly, Flood find it harder to get a line of fire at you and often end up hitting the corner if you keep close to the wall.
Beware of needles homing in on you; move around or briefly dodge out of sight to avoid them. If enemies are getting close, maybe edge back into cover a bit so you're not exposed to fire from more than one at once. However, if they're getting that close, it's maybe time to use a grenade or shotgun to try to stay in control. Once you start getting overrun it can be hard to recover.
If enemies are starting to crowd you near the door, it can be a good idea to get past them (maybe jumping over them) to get more room in the opposite side. When I do this I'll often be reloading as I go, unless I'm mowing them down with gunfire to create a path. Sometimes you may need to go back and forth a bit, to keep giving yourself room. If you anticipate their leaps, you can potentially make your move as they head to where you were, so you end up with extra distance between you. Another idea: if you've got a grenade, you might consider dropping it among the mass then moving clear.
Inevitably you'll get into desperate situations sometimes, seemingly doomed to become Flood fodder. But don't give up too easily! With good movement and efficient weapon use (picking your targets rather than shooting wildly), you might be surprised at the scrapes you can get out of if you keep your cool. Oh, and by the way, when you get forced back near the door, watch out that you don't end up stepping back out of it! Been there, done that…