Ok, I just managed to push back an erebus using the AA reg. It was not cheap to do it - I used 2 small health potions from the artifact shop. I managed to kill off his minion horde twice, but he was able to batswarm away in the end - he left because I assume he was out of mana. My loadout was mage, ring, crit gloves, decent armor, and I had nature's reckoning. I'm pretty sure he didn't have horn, but I was doing about 700 dmg per shot so maybe it wouldn't have mattered much. I think he was up to 8k health by lvl 20 too.
Killing the minions is one thing, but forget killing erebus if you don't have some serious chain-stunning going on.