On one particular game vs an erebus, I had stacked Blood of the Fallen, Narmoths Ring, Mageslayer, Bloodstone Ring, Nimroths Armor, Groffling Warplate. That is pure HP + armor + dps lifesteal. I was still unable to get the erebus's hp down, and his minions toasted me really fast, even with all that armor and lifesteal. It was a stupid game on exile fortress (no flags no gold weird map), and he still had enough gold to buy giants so he can't have spent money on crazy items. There have been other games where I have purchased a mageslayer with rook (with gods strength, he gains the most from this weapon than any other demigod) and was unable to damage the erebus with him just standing there.
These players don't use any kind of skill. They don't attempt to avoid slams. They just stand right next to me, in one spot, no mist, and spam bite with minions attacking you, and they win. By the time you realise your hp is going down so fast and need to portal, they will interrupt it with stun. If you fake cast the portal to make them waste the stun, you will be dead before you get to cast again. If you take a potion, in the amount of time it takes to use, the minions and bite will have destroyed your hp back down again.
Maybe the cooldown on bite needs to be increased? With a cooldown flag taking it to 5 seconds, thats a lot of heals (775 + bishop heal + 775) over 10 seconds. It negates a hammer slam, and his armor + hp regen + lifesteal prevent autoattack damage.
I think it could just be a probelm with rook though. It is easy for erebus to escape from a rook, so if you don't get a kill shot with slam, he's outta there. He can also kite easily and have minions attacking you whilst taking no damage himself. An UB with spit + ooze + move speed with a hp + armor build can waste a minion erebus.