How do you seal a piece of sharp metal?
Like this.
If the First had a way to fight angels, she would know, after all, that is a very big detail for her to allow herself to forget, or for said weapons to be lost (then again, to make such weapons, you need time you wouldn't have, a war with an advantage like that would be won in a week, Ilias just have to avoid the place the First is).
I think by weapons that were sealed away, they're talking about the weapons from Mitra Sealed Castle.
Basically they're legendary weapons that were created from Ilias and Alice I's energies that were sealed away in this castle. When the war started, they were unsealed, and both sides used them against each other, but they've since been lost to time, save for the remains of this ancient castle that's appeared here due to time distortions.
Or the angels are incorporeal (like ghosts and things pass through them), or not.
Nah, they just can't be hurt unless you use holy energy against them. That's why Luka (a half fallen-angel) can harm them, and why there's no sub-plot where the Enrikan fallen angels pull pranks on the dark elves by phasing through walls. The tension comes from the fact that Luka is just one guy, and is unable to save each and every person in danger, all while dealing with the existential crisis that his lineage will cause him to evaporate over time.
And if it was possible in this world, then there's zero way to fight back. Fine, luka can, but he can't be everywhere at the same time. What prevents them to pass through walls and kill all of the major leaders, including Alice all at once?
So the reason why Ilias doesn't just skip to the end and just replace all of humanity from the get-go is that she wants to try the "diplomatic" approach first. She believes that Luka will make the "right choice" in the end by killing Alice, and so she lets him go on this adventure to become a hero while getting stronger, even allowing him to learn Cursed Swordsman skills without pointing it out. Basically, she takes a mosty hands-off approach to test Luka to see if humanity is worth salvaging (for the time being). But then after Luka spared the snek, Ilias went back to her initial plan of replacing humanity with replicants, playing 4D chess with Black Alice and Promestein so that she would ascend and become a being of light and darkness. And to do that, she used war as a pretense and had her forces attack several locations in the world; she had no concerns that Luka would lose (giving him the window to defeat Promestein and Black Alice when the time came), and she'd lose nothing if humanity were to succumb to that assault, so it was a win-win in her view. But like any time that Ilias makes assumptions, it wound up biting her where it hurts, because Luka was not only able to fend off most of her assault, but humans and monsters were able to find a way to counter the holy magic enveloping the world, and worked together to defeat all the revived angels during the 3rd assault (shout-out to Frederika Mk 3, lol).
All that to say, Ilias has a lot of plans, but her expectations always fall short because she keeps underestimating other people's tenacity due to her ego refusing to allow her to admit that she doesn't know what's best for the world (whether it's besting Alice I in gene manipulation, connecting with her daughters, or understanding the machinations of her grandson's mind). Hence why Mikaela was able to clown on her twice during the final battle with Sword of Heaven.
