MC seems to be really stupid

Also they prepared a trap for the boss inside the city even with the surprise attack right after they met. they also cooperate very well for people that probably barely know each other.
They fought outside the city. Rare just dropped off her elite undead to capture the city while she goes to personally deal and enjoy a fight with the players outside the gate.
As others have said, she's just reckless and continuously pumped up right now. Even without the in-game balance mechanic of being literally drunk on power, she had plenty of proof of power gap. This series actually lays out the events and coincidences so well it doesn't feel like an asspull or idiot ball trope when the players actually make this a typical overwhelming but doable boss fight.
Let's recap:
#1- in the 1st event she slaughtered everyone without taking any damage.
#2- her dungeon that's supposed to attract powerful players greedy for her strength just toys with them so easily.
#3- she explored & unlocked a clearly late game area and mechanic(reincarnation).
#4- the greater philosopher stone even reincarnates twice and requires parts of a clearly world unique being(the world tree) and even a world announcement is made so clearly she's way ahead.
#5- she overwhelming destroyed over 3 cities that are defended by players who expected an attack and even crushed the country's army in La Colline without even using her elite units.
Now here comes the problems as there will always be counterpart blindspots:
#1- this event was too early for players to make significant specialization like scouts getting true sight. It also doesn't reflect the hidden strength and strategies of the local npcs who've lived there this whole time, like the royal debuff artifacts and emergency instant messenger insomnia tactic.
#2- it made her spend her time researching dungeon making, resource gathering and crafting skills instead of other players' skill progress. She was already confident in her martial arts and saw no need for melee skills, so being in a command & management position made it less appealing. This made her completely miss out on healing that requires bare-handed skills and others that require frontline teamplay. Her dungeon also ended up quickly pivoting to attracting beginners instead since as Rare intended, it had a consistent win-win profit for both sides. The other top players eventually went off to explore elsewhere.
#3- there are plenty of other players that explored other areas and found useful low-level items/mechanics like squid ink from the sea for blinding. They also explored combinations like the blind debuff simulating darkness and increasing the chance for fear debuff that works better at night.
#4- Rare subjugated the low-level undead around her as subordinates but others farmed them for crafting another low level but useful item she's unaware of. For example, the soul stones that can be used as replacement for inflicting debuffs on soulless targets...like undead that the 2nd event is announced to be abundant of. Rare herself is highly resistant to debuffs thanks to her horns so while this won't work on her, this works on her armor that completely relied on its own undead/homunculus/golem racial characteristic to resist.
#5- after intercepting a letter pigeon, she discovered the army was for subjugating her and felt even more hyped for the capital. While simultaneously not wanting structural damage this time and also wanting a personal opportunity to finally enjoy a fight in the event, she wasted her castling skill to rush in faster while leaving her insects behind. This leads to her coming mostly alone...straight into the debuff field miraculously prepared just in time by both luck and skill of the prime minister & the players.
Now, her excitement, mild chunii and martial pride all combined makes her act like a typical npc boss that won't back down. Look at it from the player's pov. After Wayne got lore dropped by the prime minister and apparently unlocked a hidden boss fight, Rare arrived just on time after they prepared, and alone too. She actually jumped in front of them on her own then just stood there at the beginning letting the artifact affect her, like she was waiting for a conversation prompt or perhaps it was a reward for spotting her invisibility. Despite mentioning a field debuff, she still didn't leave. She had a hit-herself mechanic that's difficult to trigger but dealt the most damage like old school games. The 2nd form transformation came with a bright flash to "pause" the fight so they won't interrupt it. It even reset her debuffs and she's now debuff immune, but still continues the fight on the ground the same anyway.
It's fun to imagine if a softer Rare went the Spirit King route and arrived armorless to talk instead of keeping quiet. With even the artifact not having any effect, further showing how "holy" she is, how many players would change sides to simp?