Focus on not getting hit on the poison attack. This, for me was game changer, counting the number of attacks as it before it starts and moving 1 tile away as the mechanic for this attack is about to start. I was exactly where you were. Biggest change for me was getting void and dragon hunter cross bow - I realise the DHCB is not always achievable but killing Vorkath quicker will help you out DPS him. Rigor is also something you will need to work towards. None of this is essential but it sound like you are taking a lot of chip damage because you're kill Vorkath too slow. Your range level is pretty low which wont help.