This is the self destruct from the alien POV, you can't really know what happens in the backroom but you should be able to see everything on this image once you've started attacking the defences.
Essentially 90% of the time marines are gonna pack into the SD and make some barricades facing the aliens and some sentry turrets behind them and take advantage of the five-tile-long chokepoint to destroy aliens that are on their way to attacking the line of barricades.
This is effective because, well, there's no way around this chokepoint. They can't melt the outer hull at the sides, they have to charge down it and then back out, which will get them shredded by the autocannons as well as whatever grab bag of tricks the marines provide, whether fire, grenades, or just a regular old bullet storm.
So how do you defeat the turrets? Let's look at it caste by caste, from most useful to least useful.
You're THE hard counter to marine entrenchment and fortification, if the xenos lack even a single boiler their job is much harder. Before sieging the SD defence head on, make sure you melt the walls and airlocks guarding its entrance to give the aliens as much mobility as possible when attacking. Once you're done, set yourself up at the back and begin nonstop launching globs of gas at the front lines. Gas has a shit ton of reasons why it's the single best tool the xeons have to offer.
- It breaks line of sight between turrets and any xenos in the gas, making the turrets unable to shoot.
- It directly damages barricades that it spreads to.
- It forces marines away from the front barricades, preventing them from shooting back at the aliens or repairing the barricades.
As the queen, your main job is to coordinate the attack. There is one order that you HAVE to give:
Order the T1s and T2s (except carrier) to stay away from the siege, they do far more harm than good. This will be explained later in their own section but T1s/T2s are unironically the main reason why xenos fail to siege the SD 80% of the time.
You'll also want to order xenos to stop blocking the crusher's charges, tell them when you're going to screech, make sure the strongest xenos (including you) are emitting different pheromones, etc., Once you're done that, start screeching regularly; this knocks down marines, but not sentries, so you'll only have a few seconds before you become yellow paste. Take the precious few seconds you have, to melt or slash the defences. Give yourself breathing room to pull out early, or a runner will inevitably bodyblock you into your untimely death. Note that the marines have to SEE you to knock them down, if you're hidden in gas, screeching does NOTHING except alert the marines that they can hug the cades now without risking being knocked over and killed.
Crush through the hull of the middle lane of the reactor room and just keep assaulting the barricades. You're not immune to the autocannons, but you're the most resilient xeno to them. Just keep crushing over and over, nothing to it, feel free to slash the barricade you're assaulting as well if you got the health to burn. If a xeno is in crit or being grenade stunlocked, it's your job to pull them to safety. Otherwise just keep crushing.
You can destroy barricades the fastest of any xeno, but will absolutely melt to any concentrated armor piercing gunfire (autocannons, smartguns) so you need to only attack when there's good gas cover or another xeno is tanking shots, and charge away as quick as possible. You're also fast and somewhat resilient enough to quickly pull crit xenos back, so you'll want to do that as well. Otherwise, stay out of the fight. You also have a bit more reason to attack when the marines are using flame.
You're more armored than the ravager, but slower with really bad slash damage, and your spit is worthless against barricades. Keep your pheromones up, melt barricades when they're gassed, and pull back crit xenos, but otherwise STAY OUT OF THE WAY, at best you're only blocking more efficient xenos from attacking the barricades, at worst you're blocking them from retreating because you're a big T3 so they can't push you away, and you're SLOW so you can't move out of the way either.
Carrier: honorable T1/T2 mention
You're actually decently useful when there's gas cover or right after a screech, as you can throw huggers into the marine lines which usually don't die because of the sheer chaos, resulting in a multi minute stun / distracts marines shaking up the hugged marine. Other than that, once you've expended your huggers, you are just another T1/T2.
Why T1s/T2s do more harm than good:
- You have no health
- You have no slash damage (except hunters)
- You have no speed (except runners/hunters)
- You have no/weak acid (except spitters)
- You have no useful abilities
YOU BODYBLOCK BETTER XENOS, WHICH KILLS THEM/MAKES THEM USELESS/WASTES THEIR TIME AND DELAYS OVERRUNNING THE SD
Every time a young runner, mature sentinel, even ancient hunter attacks the barricades they are fucking their team over. There's no other way for me to put it after watching dozens of SD defences. If I could hit a button when I reached the SD as a queen that exploded every T1/T2, I would hit it without question. So anyway:
At BEST, if you attack the lines AHEAD of the other xenos as a T1/T2 you simply wasted the gas/screech for this cycle by preventing the actual useful damage dealing/melting T3s from attacking the barricades so that you could feel good about getting those two hits into the barricades as a young drone. You'll often land into crit before reaching safety too, making other xenos retrieve and pull you to safety, which either makes them bodyblock even more xenos doing this, or if they're T3s themselves, makes them take damage delaying their next attack. Often the xeno pulling you lands into crit itself, which just cascades the problem even more that wouldn't exist if you just kept your claws to yourself and stayed behind the lines.
But usually you do even worse than that:
Far too often, if you arrive at the lines AFTER the T3s, you are PHYSICALLY BLOCKING THEM FROM MOVING (bodyblocking). T3s under fire from turrets sometimes have a margin of less than a second to pull back after they slash/melt/crush/etc, if they fall into crit while attacking they are pretty much guaranteed to die. Every bullshit young runner and mature hivelord going to the front because they want to feel useful, is one more xeno that needs to be pushed aside by a T3 that wants to leave before it gets popped, as well as one more xeno standing between a T3 in danger of crit and the rav/queen/prae/etc that can save it by pulling it back without dying itself.
So instead, what can you do to be useful?
- Wait until there's a breach in the lines and, at an appropriate moment, rush into the back of the SD to distract and kill the defending marines and turrets that aren't on 360. This is actually where you're the most useful, and often starts a chain reaction of everyone piling in to slaughter the room that otherwise wouldn't happen because xenos are too scared of dying alone without effect.
- Suicide charge the front lines to shower the marines in acid as you explode to the autocannons.
- Expanding on the above, suicide charge them to tank the autocannon bursts before a bigger T3 comes along. IMPORTANT: do NOT retreat when you're low health, keep standing there so that you explode, because if you fall into crit you're going to waste other xenos' time/lives dragging you back when the whole point was for you to give the T3s extra time by tanking damage for them.
- At APPROPRIATE moments, ie when the T3s aren't all scrambling over each other to escape, go up and weed the hall, or melt a barricade at the side, or pull back a crit xeno that's already out of visual range of the defenders.
- Stay young as a runner/sent/spitter/hunter so that you can evolve when a T3 slot opens up. Stay alive as a drone for when the queen dies.
- Just ghost and watch the SD defence instead.