You have given almost Superman-level defenses to your aliens. You say:
It has skin that bounces off melee weapons, it is bulletproof against small and medium caliber bullets, blast-proof, resistant to fire, can last extremely long without oxygen etc.
So based on that, the average home resident is going to die. In the US, some significant percentage of people will have access to firearms. But those will be small or maybe medium caliber rounds. Humans might have melee weapons, but most of those are going to be useless, as you state.
The people who try those weapons die first, as your aliens neutralize the threats before they neutralize the less threatening occupants.
Some percentage of humanity might try more drastic attacks. Chemical weapons (mace, pepper spray, bleach, etc.). Some might be cooking meals and try boiling water or something. But generally, those will be too small a group to matter, even if they succeed by some chance.
They won't (or shouldn't) have time to devise or carry out any significant military-grade counter-attacks. Not during a surprise attack in their homes. At least, not in any statistically relevant number of households.
in every household on the planet at the same point in time one such alien is materialised.
So when your attack begins, some percentage of humanity will be at work, school, or otherwise away from their household. What percentage depends on when the attack occurs (on a major religious holiday or a during a typical work day? During morning or evening in what time for your story's primary continent? Was the time optimized for destruction, or chosen randomly?).
Remember that if it goes down at 9pm at location X, that's not 9pm for most of the planet... But even if the attack occurs at 4am local time, remember that some people in that locality will be away from the house. Whether that's because they are working the night shift, or whatever...
Likewise, some percentage of humanity will be on ships, planes, or in cars. Some will be homeless. Or in hospitals/hotels. You get the idea.
Your ten percent survivors is most likely drawn almost entirely from those who were already away from home when the attack occurs. They get word of the attack, and do the smart thing -- they don't go home.
Your job, then, is to decide precisely when the attack occurs, how long it lasts, and what the aliens do next -- do they beam back up to their spaceships and leave? Or do they then move on to business addresses?
Decide those factors, then you can start to estimate how many people might have been out of the house for each timezone/city/area at that time / date.
I hesitate to throw this in, but you could take the War of the Worlds1 tactic. Some households have a common(-ish) household item that your aliens are susceptible to. Perhaps it is a flowering plant, dog fur, wheat, rice, etc. or a cleaning agent. Or black lights (yay dorm room decorations2). Or tobacco smoke.
Whatever the MacGuffin is, make your aliens completely weakened by that thing.
I don't personally like that tactic, but it works. Especially if the thing is something that seems relatively harmless to humanity, but some super-advanced aliens might not have considered in their defenses.
he quickly discovers that all the Martians have been killed by an onslaught of earthly pathogens, to which they had no immunity: "slain, after all man's devices had failed, by the humblest things that God, in his wisdom, has put upon this earth" wikipedia
For the purposes of this invasion, do dorm rooms count as households? Hotels? Campgrounds? Army bases? Here's another potential "away from home" pool, if they don't count. How you define the alien's attack vector matters.