If your current catacombs map has "Soften the Sneaky Shell" as an active objective, that's probably what it is. They have
a ton of life and are effectively physical immune, only taking worthwhile damage from charms/elemental damage.
You can usually reach out in the
catacombs subforum for help (notice all the people asking for Sneaky help).
You can log out and log back in to reset the map. You'll lose any progress, but at least not have to deal with finding a way to kill it.
This post was edited by LethaL. on Nov 8 2021 06:20pm