Sunken City (Vanadis Dungeon) is a two-level dungeon that can be accessed after unlocking Chapter 3.
You can also enter it by talking to the Soldier behind the hut.
Level 1 (van_dun01)
Level 2 (van_dun02)
To survive the Clam Soldiers and the Clam Recruits, you will need 100% Water and Fire resistances along with 100 MDEF.
To counter Shell King, you need 100% Neutral and Fire resistances and 100 MDEF. Use cloaking then attack. Cloak right after attacking, and keep repeating the procedure.