[Exxy]

An ice mountain would be a pretty cool niche idea; having the stairs go "up" in this dungeon instead of down.

Ideas for Normal Mobs
-As for the Spiders, I would love to see something like -1 Move per basic they manage to perform on you that would last around 2 Turns (refreshable on hit with a cap or -3 or -4).
-All would receive heavy Resists to ice.
-I'm tempted to say a few more mob ideas are needed.

-Can't think of much at the time for the other types you listed. . .

Thoughts on the Boss Monster

-I'm 100% fine with it being some ice-turtle-thing.
-I would not like to see it inflicting Slow.
-I would be game to see it perform an attack similar to Ice Point (although en masse) that would be a bit more annoying to deal with.
-I would also like to see some type of effect take place, either in the boss fight or something that persists through the whole instance's fights (like taking hail damage equal to LV*5 per turn).
-I would like to see it have a move similar to Wraithguard that allows it to tank more damage (possibly refer to it as Withdraw) or to have the mechanic be a Passive Skill that slowly builds up over time.
-Possibly have it follow other similar boss tropes (I.E. Give it Demon Translation or some skill similar).

Final Thoughts (Edited In)

-By no means would I like for this to be an "easy" dungeon (and I 100% expect people to complain about for some reason the things I listed would make them too OP. . . 1, not all are meant to pass through but I would love to see all do so.[strike]2, Suck it up Koonie/Airde.