So there's this issue where pretty much anything in the erebus will one-shot you. If you test out in single player, things hit normally (also on Kappa) and I asked around the PO3 discord, who also thought it was weird.
A possible explanation is the Epic Siege Mod, which makes mobs more powerful for each boss kill- which is great, when it's just you killing bosses, but then you have a server? And it's just too much once a few people actually farm a hundred or three.
I believe this may be fixed in the config for epic siege, under "The factor by which mob health and damage multipliers will be increased when bosses are killed".