They're completely resistant to slashing..which means any slashing weapon wont do anything to it. The slimes on the floor below are resistant to blunt damage, on the floor above there are skeletons which are resistant to piercing, and the weapons on the top floor are resistant to magic.
The mobs in Appolyon SE are not just simply weak / strong to their normal opposites. They just have very polar defense and weakness. Ranged attacks, polearms and daggers work very well on these. Katanas hit for 0.
Found in apollyon SE. They are, like all mobs in this area, weak to what normal crabs are strong to. That means that its time to break out that hand-to-hand gear... (and magic still works just the same)
A lot of non-blunt weapons will hit for 0 dmg... Im still not sure what exactly will work and what doesnt. All i know is that Great Katana and Katana did not do any damage. When the SAMs swapped to polearm they were able to damage it though. Ranged attacks DO work, however leviathan's spinning dive does not and both are considered piercing damage as far as i know.
These guys can take a long time to kill so be prepared to spend some time fighting them if you dont have a lot of monks or blms in your group.