You are dying horribly because of a number of things. Poor gear and missing 1-2 attribute changing item per character (bad party setup as a result).
Here is the full list. Go back to etigoya's guide and search for important items (重要). You need to gather them all for post-game dungeon anyway
Also, revisit the towns, dream world, and also some dungeon worlds sell gears.
The robot below sell an armor that boost 100HP, elemental resistance and also the black miasma that the boss uses in the 2nd form on. (Go east after using the right teleport stone at the beginning of the dungeon).
This armor would make the boss fight a lot easier.
This lady in the dreamworld sells an underwear that protects you from tentacle attack... but its harmless here, in fact you are better off using instant death protection.
Both the white robot in base and stores have better gears for you as the story progress.
First option is let's you combine normal store bought items into stronger ones.
Second option combine boss + mob drops into stronger equipment.
Third one lets you combine all the sperm you have been collecting into stats boosting items.
Craft some
from first option to protect you from instant death.
The MX armor (see attachment) also have instant death protection, but for that last dungeon boss, the armor sold by the robot NPC (2nd picture above) in the dungeon offers better protection.
For the black hair girl, give her these 2 to reduce her MP burn:
Lastly, complete other events (e.g. the monk in your sister's store is sending you to another urban legend kill quest).
