heyy, i have done the quest! I'd recommend to not make the walkthrough a holy grail because there's A LOT of difference there compared to the latest (0.66) version.
SPOILER ALERT
first, you need to already have bounce skill from the Seal Of Wind's chapel. Then, after finishing the seal of wind and talking to Lysia, you need to hop around her house and read the bookshelves or spread out papers. The goal is for her to make an incantation that doesn't kill her in gruesome ways. Hints like how oxygen is reactive and rubber tree causes transformation instead of simply inflating.
The ones that I do is:
Puffshroom, Helium, and polymere leaf for herbs (I forgot the exact name of the leaf, but there's a hint that this leaf stabilizes reactions on the bottom right bookshelf.
Then use Ventus, Stabilium, and Crescende for the incantation. This too can be seen on the bottom right of the same bookshelf.
Other runes do other things. Latex, for example, turn Lysia into an actual dead balloon. Explodere will outright rupture the girl, and not in a cartoony fashion. Combining it, and you will have an exploding dead balloon.
then, after inflating her (or killing her), you get the key to go to the cave right at the bottom of the map, grab the two blue orbs within the cave, put the orbs on the big gate, and fight the boss.
NB: you will get a key to the boss battle room regardless of what you did to Lysia. I also do this in the so-called vore route, pacifist route may do other things to her, possibly recruiting the girl or something.