As discussed regarding Lokkestiiz, there are a few notable differences in the strats that purely progression and endgame progression groups have utilized for Trials. Further pointed out by fellow ESOU Professor JerBearthe3rd, main tanks going under Yolnahkriin is an important part of conquering the fight particularly on the hard mode setting.
The primary reason for rolling under Yolnahkriin and then stacking the two Flame Atronachs that spawn with every Iron Atronach is, of course, DPS.
Many progression groups have the offtank hold the Flame Atronachs away from the rest of the group so that they can later be focused down after Yolnahkriin is pushed to flight phase at 75%, 50%, 25%. While this is fine on non-hard mode, it's a bit sketchy on hard mode where the Flame Atronachs "ignite" or enrage putting everyone at risk.
Most groups starting out then focus the Flame Atronachs before the flight phase but this means less DPS on Yolnahkriin opening up the risk of getting another Iron Atronach and another two Flame Atronachs depending on the group's DPS.
The most efficient solution immediately became putting the Flame Atronachs under Yolnahkriin (usually by the main tank who's being Guarded by the group healer) which meant the groups could simply cleave the Flame Atronachs safely while focusing their single target DPS on Yolnahkriin avoiding the next set of add spawns.
- Roll under Yolnahkriin from a certain distance (as pointed out in the above video) and to a certain distance (also pointed out in the above video).
- Prioritize taunting the right Flame Atronach immediately and then the one closest to you (to your left behind). These Flame Atronachs ALWAYS spawn in the SAME locations. Sometimes the fight bugs out where only one may spawn.
- First person perspective helps A LOT while under Yolnahkriin in targeting the Flame Atronachs and pulling them whether it's with Silver Leash or Unrelenting Grip.
- Block-casting while under Yolnahkriin will mitigate the Fire Geysers the Flame Atronachs may put on you and the circular AOE under the dragon.
- Roll out after. Depending on when you come out, Yolnahkriin will attempt to Heavy Attack/Bite you, start a Flame Breath channel or start a Flare.