What you need is:
Familiar - Ancient Guardian makes it so whenever you drop below 35% health it absorbs 1 attack every 6 seconds.
REMOVE ALL youre life/sec gear, then bring ur hp below 35% of ur max (That's easily done with unequipping items and then equip them back on!)
For example you would go to the boss like this: 4721/30000
Your HP will never gonna go up
Then go FULL dps spec:
Magic Weapon with Venom
Teleport Wormhole
Storm Armor with the Rune to make you run faster on hit
Familiar with ancient Guardian ofc
Arcane Orb with Obliteration
and Venom Hydra
For passive you should go with glasscannon, evocation and critical mass
Tactic:
At the beginning he will teleport to you, but he can't kill you because your Familiar will absorb it.
Before he hits you, you can use hydra and you can just run 25% faster with storm armor or just blink away
IMPORTANT is, NEVER stay close to him otherwise he will summon his minions
Just go max range and he will teleport to you again and then you just repeat the steps at the beginning of the fight!
Happy Farming