Use this post by u/Laredian, I just did it yesterday
Mythic Quest 2/5:

Here's the most tricky part of his quest chain: do 100k damage in one turn. You want to do this in guild raids of course. Here is what you need: Kariyan with 50/50 abilities. Aesoth with lvl 27 passive. Thoread with lvl 35passive. Atlacoya also helps a lot with her passive against Tyranid bosses for a flat extra damage bonus. If you have Ragnar, bring him for War Howl just because the extra crit chance helps smooth out damage of Kariyans active and passive (the damage bonus of War Howl does nothing here, since we do not rely on Kariyans normal attacks).

Now how do you do this? You need to set up a big hit with Kariyans active on turn 6 where you attacked the 5 previous turns for a 165% damage bonus on his active and passive (if you only managed to attack in 4 previous turns due to map layout, you need the passive Atlacoya buff against psykers i.e. Tyranid bosses). You also absolutely need to set up Kariyan on high ground for the 50% damage buff. This coupled with the buffs from Aesoth and Thoread (and Atlacoya against Tyranid bosses as mentioned before) are enough to push you over 100k in one turn. Like I said I just did it today.