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Okay, after a bit of reviewing my notes, it seems the only place left to go is Pazuzu's Tower. 
Unfortunately, the entrance nearest to where I left off just loops back to itself after forcing me to fight some extremely rude birds; Mucus and Hydro Acid are okay, Blackness... well, hurts a bit less but blinds (and I can't afford to miss), and they can also petrify. Well, off to the inn, then back to... some other floor, pretty sure there's somewhere I didn't go due to the enemies.
This is, in itself, a very good reason to not go to the inn very often right now. Unfortunately, I'm pretty sure I'll need to White something pretty soon, and Phoebe doesn't know it yet.
Well screw you too.
After dying to regenerate the fight, I get just these guys... and that is a hell of a lot of damage, even if it's a crit. Also, they're faster than me, so I don't usually get a turn. Thankfully they also have a rather inaccurate Rapier, so there's a possibility that I won't get wiped before getting a turn.
Unfortunately, they seem to prefer Psychshield (the 3000-on-crit attack) over Rapier, and to add insult to annihilation they also have Cure.
Maybe these guys will be less OP? I restart the fight a few times, but apparently this is the only formation that exists for the Thanatos leader.
Well, they are, in fact, less OP (despite me seeing yet another Dark King attack), but that doesn't mean they're possible. And I can't even use White because that Beholder reflects magic.
...Guess I need to do a LOT of level grinding.
Okay, now that I'm level 17 I might actually have a chance of getting a turn for a change. Let's see if that actually helps any.
Well, it takes a lot of banging my head against a metaphorical wall, and a bit of desperation to try attacking with the claw, but I eventually manage to paralyze these Quake-spammers to death.
That's a lot of doors to check. Let's go to the one to the right first.
It's the overhang above the Wintry Cave's boss room... oh hey, finally got a defense upgrade! 
And the River Coin right next to it, so I can actually explore some new (and hopefully less unfriendly) territory soon!
The bottom-left door in that Giant Tree room puts me at the Wintry Cave entrance, where I find... a bad spell. Second-worst in the game, in my opinion.
Phoebe has nothing.
The top-center door in the Wintry Cave entrance leads to the Giant Tree top; I might come back later, if I need to open the door the boss blocks.
I also find the Sun Coin for more region-unlocking.
The local skeletons love spamming Doom Dance.
I wouldn't've bothered fighting them if they weren't standing in front of this chest.
I jumped down the top-right pit in the Tree so I could get at the northeast door on 4F more easily. Can't say I'm pleased with the result.
It does lead me to more Bone Dungeon, though; it is, at least, pleasing to be able to dominate enemies for a change.
This is nice, except now I have to get back out with it. ...and can I just point out how weird it feels to be faster than Phoebe?
Continuing past that room, I find the skull end of that skeleton I couldn't get through before... sweet, my first tier 2 weapon- now I won't be quite so worthless if I have to actually attack stuff! Now if I just had some more armor...
Also, it turns out that getting out was actually pretty easy; I ran into a pair of skeletons (instead of a group of 3) and blew them away with Flare.
Y'know what, I'm doing well enough for myself now that I think I'll try killing these guys to get at another red chest.
Well, that worked, but... Y'know how I said Quake was the second-worst spell? Here's the worst; there is literally no reason to cast it when you've got access to Life.
It takes a bit of doing, but the northwest stairs in Pazuzu 4F go... Bone Dungeon again? What, is the whole thing in here?!
I found all the Coins in this one dungeon. What the hell.
Oh, that explains that. Off to the inn again so I can explore that last staircase I can access in Pazuzu!
Back to the Ice Pyramid, as it turns out.
The staircase next to the grating in the middle (on its right) just goes back to the top of Pazuzu; there's nothing here for me even if I had the Dragon Claw.
The one on the other side takes me to Spencer. He has nothing.
And the southwest one loops back to here.
The scaircase just south of there... how the hell big is this dungeon anyway?! ...Questions for later- I need the Dragon Claw to go any further in here.
I'll see what shenanigans I get dragged into over here next time.
...holy crap I had to literally double my level to get through this update. These settings are EVIL.