Ouch! Thank you for sharing this experience. I was planning to make my second character as summoner but now, after reading your post, I have my doubts.
I was using Livor Mortis as Shadow Warrior with my icy mage build. Sort of helping hand/tank buddy and it worked well against crowded ads which that boss spawns. Have you tried that or have you used defult Livor Mortis?

Sorta in the same spot.
I´m using Annihilation on right mouse, Livor Mortis, Hunting Swarm (or Parasite), Bulwark of Dawn skilled as damage and Ether Vortex (don´t remember skill name atm, puprle damage pool)
In the first phase i just refresh Livor Mortis when he´s on the boss for aggro, set my bulwark on Livor Mortis (Bulwark still seems to heal) and with vortex and hunting swarm just spam for Adds or on Boss. Actually no Problems here.

In the second Phase I have the same Problems. The Boss doesnt seem to be attackable, only the pools he spawns. But minions do what they want and my damage output is to low. Got the boss for about 10% of his life pool. After 10 tries i called it a day lol

Well i was going full toxic so i kept him base and gave him mods for increased agro, damage and life. I had zombies set up as toxic as well. The build honestly feels great up until that particular fight. It had decent clear and everything. The only other problem is, some of the mob ai when you possess them with parasite seem to range from horribly bad and do nothing to pretty decent. To beat this boss i ended up changing my build completely. Once im in the end game with a few more passive points i may attempt the build again as i feel it will probably do slightly better in the end game.

I know this is a month old, but I was able to complete the 2nd act boss with a similar build, but I lost 3 times before I got the hang of it (with a video assistance). Passive picks for toxic damage and ailments, around 50/50 split tough/willpower. My skills were aether jump, golem with mo'health, parasite, plagueburst, and annihilation (toxic damage) on right mouse.

Most of the fight is about knowing the boss and practicing; again, I'm not good, so I failed 3 times before getting the hang of it. The keys the fourth time for me:

1) Save plagueburst for when the boss dumps minions. You goal is to pop them fast, hopefully grabbing one with parasite (healing yourself in the process from incidental damage). I think the gas and explosions from the plagueburst really helped.
2) Aether jump to the magic glow circle of safety (whatever) and hold your ground. You're dead if you're not there.
3) Stay pretty close to him - I think the poison within 7' node helped, and the bounces from the staff.
4) Only use annhilation during the slowdowns, but do use it.

-Jeff