Few stories can make me feel such sadness in just a few panels.
We go from a kuchisake-onna obliterating an entire alien fleet... to her memories of a happier past, before the evilness of humanity reached a new peak, and then... she just leaves, because two kids showed a love that she didn't forget but she doesn't have anymore.
This series is special.
About how she could win so easily... I am inclined to believe that the "Empty Space" the aliens have to enter somehow serves as an amplifier for ghosts, Reiko is already a high-level ghost, so her power becomes even greater inside it, I say that because... this feat of imprisoning an ENTIRE ALIEN FLEET inside a mirror space is not small, and if she can do that easily on "Real Space"... this kinda breaks the power dynamics.
But I do think is because of "Empty Space" she could do that, which would easily explain Okarun's theory that the fact aliens don't invade Earth en masse is because of the ghosts, whatever is the "system" that forces aliens inside that parallel space works best for the ghost and they can't break the system easily.