Just save energy overload until max for this fight at 1000/1000.
AOE the portal
The start light attacking the boss only
When you see it grab the power sigil asap
The boss will summon 1 add, then 2 or something. Then he will summon 3 and 3 again super quickly, by now you should have him closeish to or in execute range, grab the defense sigil so you can reflect the ranged adds and kite the melee ones.
Kill him before he withdraws the 6 adds or he will heal himself and your overload will be almost used up
Just focus the boss and ignore the adds, this is a DPS race.

You're a Stam Sorc, so just grab some IC jewellery, re-enchant for damage, use DW, max crit and a ward going (just do it ). This is at least how magicka is going about it, I imagine steel tornado is going to work well on the adds, so you could use the Nb or temp technique and kill the adds.