Mythic bosses are just overtuned, it's neither fun nor engaging.
I've done everything correctly for the mythic bosses, suffered through grinding my raid team to A2, got mythic biovore, watched replays to not waste tokens because there is still no practice mode and one wrong move can ruin your run and put your guild behind.
And still Laviscus dies a turn too early just standing next to Mortarion. I don't know how all the guys in the replays do it to keep him alive. I am using the exact same positioning. Do they all have their characters at mythic wings because they chucked thousands of dollars at the game until they got lucky getting all the mythic shards they needed?
I know you are not supposed to survive Mortarion until the very last turn of the attempt and thats fine I guess, your run is planned around that fact anyway (using Waagh a turn earlier than you normally would etc.). But if I have a full A2 team (and even got Atlacoya to D3 as a flex slot) I'd expect my core pieces to at least survive to the second to last turn. If the requirement additionally becomes that your characters have to be mythic wings or one step before that so you can execute a run properly (against the first mythic boss no less, primarch or not), then screw this.
Mythic bosses health and damage is overtuned and it is neither fun nor engaging. Also putting Lion, Mortarion and Magnus back to back to back is just asinine. Do better Snowprint.
And for all the Snowprint bootlickers in this sub: no, I am not saying mythic bosses should be a cakewalk, but there is some bandwidth between overtuned and too easy.
Edit: I forgot to mention that I even got Kariyan at three blue stars (I'm still 50 shards away from wings) and in my last attempt he only barely survived to the second to last turn because of his resilient trait, just because he stood next to Mortarion. Either reduce the health pool of the mythic bosses or tweak the damage, because this is just stupid.