I assume your pet is tanking. Try using a pet that can lust if you aren't doing that already: there are non-exotic pets that do this such as nether rays.
If you aren't, use full consumables: rune, food, flask. Use the BRAWLER'S guild potion since it gives 3000 stats while the normal prolonged power potion gives just 2500. Remember to pre-pot and do NOT delay the 2nd-potion or else you will lose a lot of it to enrage.
One tip I used was standing next to the boss during Kormrok phase so that when I got targeted by the hand I could continue focusing the boss down with single-target while passive splash damage got the hand down. Of course this is depending on what your health allows. I did this as BM where my Dire Beast Stomp talent would get the hand down, but I'd imagine you could do it in a similar fashion as MM with Piercing Shot/Explosive Shot.
Consider gear/legendary/talent choices that are more beneficial for 2-minute pure single target fights.
5-8% is within the realm of killing it given good RNG. You could also come back after getting one or two good gear upgrades and probably kill it a lot easier.
Oh, and make sure you are using the best gems/enchants available: that means no +100 gems or stuff like that. Use the expensive stuff.
Finally got him down. Used drums at the start. Trickshot. Patient Sniper, Crows, Turtle pet to tank, double brawlers prolonged pots. Had plenty time left (fight took 1 min 37 seconds). Tank pet and drums at start really helped. I had been using lone Wolf and drumming after the pillar. 678k DPS.