I just ran into an enemy that I'm dealing zero damage to
is there a way to get him to leave without having to logout and reset the catacombs?

You need to cast against him or reset catas

you need to deal enough dmg to overcome his def. ideally you crit high with powershot using a very nice heavy weapon... just saying^^

If I buy an ee charm and just swap that for my dex charm, would that be enough to kill these guys? (I have a good dagger but still not doing enough)
Ive had to reset catas like 5 times now and its kind of annoying

It's Sneaky, you cant beat him without EE Charms. If you are melee, just reset catacombs.
If you switch to EE charms, it will take hours (regen mana etc)

better ask someone to help you out with a high lvl caster^^

This should be Sneaky, he has a huge Meele defense, the only way to fast kill him is by using Caster.

In this case the options are 2, you leave and remake the game or ask for help here: https://forums.d2jsp.org/forum.php?f=274


3rd option is to msg all Casters online for an help