This dungeon took me several tries.
Even though I used the latest equipments and focused on VIT (and my VIT reaches about 260 by the last floor), the last boss (the thousand-year turtle) still deals a lot of damage that Stella would die in 2 hits.
My breakthrough was I used the accessory マタタビネクタイ, which increases the chance of encountering enemies. It may seem counterintuitive at first (more danger to Stella), but since she encounters more enemies, she actually gets to level up more and her stats rise more. Her VIT reaches 270 plus, still lower than I expected, but apparently enough that she doesn’t get killed by the last boss easily.
Well, it could also be luck ^^;
p.s., the accessories that increase stats by chance in a battle just applies to the battle where they successfully activate (i.e., not permanent) so I think they are not too useful…