humans are 1000 times stronger than monsters, even a tiny piece of metal can kill if it's shot out of a gun.

I'd say it probably has to do with them being mostly made of magic rather than matter, and maybe Chara/Frisk's hatred fueled some magic into their attacks.

they explain that in game:
monster bodies are made of magic, upside is they get to manipulate magic really easily and do crazy bullet-hell **** , downside is their body and soul are largely the same thing so attacking them physically is basically attacking their lifeforce too, when they die they just collapse into dust.
humans are sturdier, both body and soul are strong enough to persist after death. also instead of naturally having magic, humans can just sort of destroy monsters by attacking them with the intent to kill, a physical attack backed by a powerful soul is basically kryptonite to the monster's magic body.

also time-resetting anime-esque Determination to change fate ******** helps (probably not a trait all humans have though, time would have broken by now)

So in the endgame genocide, with what you said in mind, how did he "destroy the world" with a knife? The game just fades to black, unless the interpretation is that he kills the player themselves.

Not to mention the questioning of the monsters' declaration of war with this sort of information of how brutal a single human can be. Like, who had the gall to actually go against humans?

Yeah it seems like absorbing souls has like an exponential effect on their power.

I've always thought: if Asriel, a child, had the possibility to annihilate an entire village with just one soul, then if Asgore had just done as he was asked and absorbed the 6 souls Chara would never stand a chance.
The only reason you win against Omega Flowey is because the souls rebel against an evil host, and the only reason you win against God Asriel is because he starts feeling emotion and gives up. Neither of those would apply and Chara would get stomped, many times if necessary, until they gave up and died permanently. Hell even if he had like one soul I bet he'd beat Chara no problem.

It even says that in the intro, now that I remember, but I completely forgot about it... **** .

Well anyways, knowing this, wouldn't history repeat itself and cause humans to fear and attack the monsters (regardless of Frisk's ambassadorship; humans are assholes) again?

If you go to the library one of the books talks about the war between Humans and Monsters, and it says that the level of damage dealt by Humans is equivalent to how much they want to hurt their opponent, so if a Human struck a Monster with the intent to kill, like Chara...