I beat it as a mage by eventually abusinng cold blood pyroclasm to beat Feronius . I don't think any other build can get past him right now. I think the portal to Onyxia is broken, counter attacks can happen even when im not being attacked, just whenever it's the enemy's turn. Polymorph makes the target unable to act even after they break out of it, so combine this with cold blood pyroclasm, a lot of agility and it's over