It sounds like you ran into the objective "Soften the Sneaky Shell" he is immune to melee dmg.. you can use caster gear to take him down, although, it would likely take awhile to complete him. You can also find a caster to join you and take him down.
What most people do is Create a Group, then leave the group to reset their catacomb.
If you do this, you won't see him again as he won't spawn anymore and ruin your maze
E: oops I was too slow
This post was edited by JonWon on Apr 11 2022 02:19pm