The level 200 summons were never intended to be used for any quest bosses, like trululu said, they do deal a lot of damage, this will make the boss fight too easy especially if all the players summon them.
However, most of the time you usually don't fail to beat dragonking zyrtarch because you don't have enough damage but instead of his strategy taking too long.
I always recommend players to bring at least two druids for a dragonking team, why?
- Druids have Utori Pox (Envenom), when the druid cast this on any soulcatcher (the pillar in the corners) they have a 100% chance to summon a Corrupted Soul , having two druids with utori pox means you'll have two corrupted souls straight away as soon as you come back to the room!
- Wait until you have 3x Corrupted Souls, then have your friends help you trap the souleaters so they die on top of each other, a great way is also to use wild growths or magic walls to help trap them.
Dragonking Zyrtarch is for sure the one that requires the most teamplay out of all the Thais Bosses, I hope the small tips above helped you a little bit (if you didn't know about it).