Maybe your issue lies in pet AI, there are different modes, by default they are protective and lose target if owner is being damaged. Try to set aggressive AI. (sword/shield icon on pet portrait)
PS: I did beat him as sumonner.

tried using the different pet AIs. no luck... they behave the same way. as soon as the voice summons its demons my minions are all over the place and sometimes even refuse to follow the pet attack command, while i'm constantly getting paralyzed and zapped with doom bolts. i do some damage, but the voice gets those hp back everytime i die.

it seems i'm stuck and may have to completely reskill my character...

can you show his equipment? Or zip and share character?

looks like destiny "solved" my problem... after running flawlessly for almost 8 years, my pc is gone now. i'm typing this on a new one that I bought yesterday. since I don't have any backups of my stuff, I will have to replay the game :/
it will take me a while.

thanks for your help though, Gremlion. if I manage to reach that point of the game in near future, I will post again here.

Maybe share your build via https://grimcalc.com/ ?
You would be able to show character in advance

i vaguely remember items and devotion points, but i made a build with the same skills my character had:
http://grimcalc.com/build/1008-rEWkQp

a list of items i remember:
- empowered grimoire of og'napesh
- beastcaller's cowl
- black hand of sanar'siin
- two of the black legion rings with pet bonus
- the relic that gives you the storm hound

for my devotion points, i think i chose revenant, rhowan's crown, behemoth and some of the other lesser ones with pet bonus.

This would be stronger:
http://grimcalc.com/build/1008-vQLiRm
Explanation:
Occultist:
1.Aspect of the guardian doesn't give you anythng useful.
2.Maxed Blood of dreeg - more Offensive ability=more crits=more dps
3.Maxed Cuse of frailty = aoe slow, more damage taken by monsters, easy way to activate "on attack" devotions, like sheperd's call or Howl fo mogdrogen
4.bonds of bysmiel+manipulation at 1/1, have low scaling, just activate it and get bonuses from +skills items.
There is a cap on attack speed, you can only double it, most likely you would reach it with items alone. Pet damage bonus doesn't help much, since you would have ~600% on items alone, 60% from 12/12manipulation is just 10% increase, too low for 12 point investment.
Shaman:
1.Grasping vines to activate "on-attack" devotion
2.Oak skin in Mogdrogen pact to survive longer
3.Emboldening roar -same as manipulation, low effect/cost ratio

as for devotions, I would recommend some damage devotions linked to pets, like manticore to hound, and try to get Howl of mogdrogen.

thanks for your help. i will try the suggested build as soon as i get grim dawn to run smoothly on my new pc.