My quest for the flawless title on my MagSorc began about 2 weeks ago. I finished a regular vMA farm run with just 1 death after dying to the final boss as a result of my own stupidity. Needless to say, I was quite upset. Would have been nice to just more or less "accidentally" get the achievement, but it is how it is. At least this run encouraged me to go for the title by telling me that I'm capable of doing it. So I accepted the quest.
I started out by using the infamouos 1 bar pet sorc build from Mistress/Alcast or whoever else claims credit for it. Although it's a bit cheesy, this build helped me getting through my first clear about 1 year ago, so I thought if it ain't broke, don't fix it and all that stuff. You know the drill. The first few tries were a pain. The pressure of not being allowed to make mistakes had a huge impact on my play. I was shaky, overly careful trying not to miss any mechanic. I failed surprisingly often in the early stages where it was usually almost impossible to die. The build is incredibly slow. You're basically exchanging survivability for damage output, which drags out the fights and forces you to stay focused a lot longer than needed. At some point, I decided to change to my usual MagSorc setup and give it a go. Bye Plague Doctor, hello False God. Bye lightning gun, hello Crushing Shock. Just the usual cookie cutter build, taliban imp, healing bird, maelstrom staff on the backbar. I was prepared to get one-shotted by everything with this build, but to my surprise, I nuked my way through the first four arenas without any problem whatsoever. The ice arena caused me some trouble though. This is the first mile stone on the journey, there is a lot of stuff you need to anticipate, a number of mechanics you need to see and I do not just mean the last boss. At this point, I realised that the imp was not worth slotting. It died way too often and the damage output was too low. Yes, occasionally it facetanks a mob, but this is unpredictable and too random, so I dumped him for bound aegis. This setup:

Frontbar: Hardened Ward, Crystal Frags, Crushing Shock, Bound Aegis, Matriarch
Backbar: Wall of Elements (MA Lightning staff), Lightning Flood, Crit Surge, Bound Aegis, Matriarch
Iceheart, False God, Mother's Sorrow

...worked a lot better for me. The only obstacle I bumped into on a regular basis was, that my pet died and I did not notice that I lost it. Luckily, there is an AddOn that gives a huge red "PET DIED" notification.
This build finally got me through the arena after maybe 10 failed attempts, where I either got killed by unfortunate poison plant spawns in arena 7, ultimate not ready in arena 9, me being unable to jump onto the ice platform in arena 5, me uncovering 5 pilars and forgetting to uncover a second one afterwards in arena 6 (and getting swarmed by spiders) and me being to uncoordinated to block the skull on the ring platform in arena 9. Eventually though... I made it. The final round was a mess, I almost died upstairs but eventually stabilized and got things under control at the end. When I grabbed the shield sigil, I knew it was over.

So here's my advice for everyone who is about to try this with a MagSorc:

- Find your own build! Do *not* use the 1 bar build. It's a great build to start with and get you through your first clear, but more dps = faster progress = higher learning curve.
- Be prepared to die. If and when you do, log out, do something else, relax, chill. Do not try back to back runs.
- Do practice runs. Go to certain arenas/critical bosses and practice this boss. Do not kill it, just suicide when you're about to win and repeat.
- Remember the details (do not use crushing shock on the behemoth, always prioritize the targets, charge your ultimates etc etc). It's the small mistakes that will get you killed.
- Get the "Pet is dead" AddOn. It really helps. (Sorry console folks...)
- Iceheart is a life saver!
- Use the sigils and more important: remember WHEN to use them.
- Keep your shield and crit surge up (use the matriarch heal as an o-[snip] button)
- Use blue food (Health/Magicka), you don't need that much Stamina if you're movement and DPS is ok.

[Edit for minor profanity]