ToL missions - pet tanking

With my very slow Shei kill it appears that alien adds get spawned at %s: 75,50,25,20,15,10,5. While Akheva healing adds pop on a timer, although it may be a %, I just don't have a good read on that.

Another degree on uncertainty - when a healer add heals Shei, then the alien add of that % will get spawned again.
Here is what I mean: if Shei is at, say, 15 % and it just popped a datiar, then if a next akheva healer heals it so, say 22%, then datiars at 20% and 15% will be popped again. It makes adds schedule to appear random from mission to mission.

What is true in my books - Shei health locks at 2% if any of the adds is up - dps datiars or healing akhevan. With my slow grind Shei's health didn't lock at 2%, it went from 3 to 0 with no stopping, and there was no adds up at that time.

This is what I think too. Once I practice with the bard more I'll try to burn Shei again.

Healer adds being up but not killed do not lock Shei's HP. You just have to get rid of the Datiar. That might also help your burn process.

I'm pretty sure no more than three healers spawn. The initial three I'm pretty certain are timed. They have no time to get to Shei with a heavy burn. (As an unrelated side note, Centien Detreius or whatever the name is of the last Akheva in Close the Gate, can be killed before a single add wave spawns, though I've only seen the planets align for that kind of headshot DPS once so far.)

This anti pet tank mechanism is at work on named in Vex Thall. It is the only zone that is affected.

A quick update - did this mission again with the same roster. The bard did better, the mage kept casting Malo until it landed. Then spells started to land at about 50-70 % rate, which was good enough for the beastlords to dot and use Coalition. Overall group's dps went from 437K to 736K without any burns. Shei went down in 596 seconds.
Datiar (dps alien adds) are definitely %-based spawns, at 75,50,33 and then 2 more. Healer adds spawned exactly 3 times as mentioned above. In both fights (this and previous one) Shei metered at 439mil HP - that's approximately initial 330 mil HP + 3 times 10% heal + some mob regen.
All stars are aligned now - pet tanking works, melee range is entire top pad, dps adds are %-based, 3 healing adds at +10%, Shei needs to be Malo'd for spells to land.

Well thanks a lot for all your precious feedbacks !

Since my friend was off for work for a few days, I tried Shei on my own : MAG-MAG-BRD-heal-heal-heal.
I followed the main advice to stay on the wings of the platform, here's how it went :

1st round : Wasn't fast enough to clear a datiar > wipe
2nd round : Wipe right from the start as two of my heal-merc were still on the platform and got spanked by Shei.

3rd round : This time I pay much more attention to the placement of everyone. Emote Shei and engage. One of the healer get killed but then the pet get the aggro for the entire fight. I get the DT curses covered by the bard as well as the datiars (NB : one time, the cooldown of the mezz spell wasn't down and a datiar spawned. Hopefully I could hit the AA mezz in time).

It was a bit stressful not to miss those two events during the mission while boxing. I have my main mage-pet tank on a screen, my second mage and my bard on another one. So I was almost always focusing on the main mage and the bard, at the expanse of the other mage. I could clearly see that on parse log : main MAG : 186M, air pet (tank) ~80M, BARD : 48M, assist MAGE : 43M.

Shei Vinitras in 774s, 360,65M dmg @465,95K

Really stoked - but nervous - at the end of the fight ! Loved the massive loot as well as all achievements completed.

I will provide some feedback with our full group asap !

Thanks again, all your comments were really interesting and helpful !