Could someone help me kill this monster. I am level 55 and I can't hurt it at all. Is this normal?

it's likely you spawned the quest "soften the sneaky shell" (just sneaky for short) which is immune to physical attacks, generally killed very easily by a high level mage.
Not sure too many players are online at the time of your post, so you may struggle to receive assistance.

If you create a group then leave the group, it will refresh the map, so any exploration is lost, however it will also refresh the mob so you're no longer fighting him. He doesn't come up very often so it's a quick and simple way to get back to killing mobs.
Also note that sneaky can spawn as any mob, so next time could be a huntaguar or anything else.

sneaky spawns often enought, worst case in near every new cata you start. best way and to keep the chance to compleate the map is to kill him or to ask someone to help killing him, so he cannot spawn again.

Thank you both for the info. Next time I'll wait til I get help, because I keep getting him and can't completely clear any catacombs. :)

if u need help still lmk

thank you. I'll let you know when he starts smacking me around again.