Since I'm a guy, I'm not gonna tell you every detail. I'll just show you pictures and point. You'll figure it out.
Now that you are geared up, head back to Nanai.
I'll give you another slide show to navigate the first floor and find all of the treasure chests here.
Here's a slide show to the treasures.
Once you get down stairs you'll notice that you aren't in the fiery bowels of hell anymore. Head down the stairs and up through the hallway.
No, he's going to attack you, for disrupting his privacy. Of course he is.
Knock his health down to the critical level and he casts Cura on himself, he also gets some sort of "Hysterical Strength" which increases his attack power so that he deals around 10 damage per attack. He still uses his T.Bolt regularly as well.
Hang in there and keep healing. Once you get him to say "Aargh" the battle is almost over. You should only need to last another 5 or 6 turns and victory is yours.
After the win, he gets chatty.