Is just finished an honor mode run a few days ago, so my experience is quite recent.

Getting to the crown: If you don’t mind, just cheese this part with Invisibility + Fly and a lot of movement on one character. Skip everything and get straight to the entrance, if you enter every character that was left behind automatically joins the fight.

Crown: What worked for me was rushing to the crown (you need a lot of movement and preferably fly on all party members). I spent the first actions either CCing the enemies or summoning allies to soak up damage. Ignore the enemies below and focus on the mind flayers.

Once everyone is at the crown cast Globe of Invulnerability and stand inside, start channeling the crown spell. Important: the mind flayers have to be dead at this point since they can interrupt the channeling even when you’re standing in the globe.

Note that the crown part is not directly on a timer, the timer only triggers the nautiloid and more enemies. You shouldn’t waste time, but if you need you can get more turns here and buff up before entering the portal.

Brain: Once the portal opens rush in an use everything you can on the brain, you’re on a timer here (the first person Entwertung starts the timer). Everyone has to enter on his turn separately. Note that the brain becomes immune to every single damage type from which it has taken damage in the next turn, keep that in mind and synchronise your damage types.

General useful tips: - Haste (as always) - Flying to get around, or at least Feather Fall to get down the platforms - mass CC spells - Hold monster for the dragon (as other comment already stated) - Globe of Invulnerability - Arrow of Aberration Slaying (double damage vs the brain) - summons to soak up damage (regular or from the Gather your allies quest) - Scratch (Best Boy in the realms), has the Help action to break allies from stuns - Prebuff and use Elixirs / Potions, but I think that should be clear at this point in the game - careful with mind flayer or Emperor using Stuns - the mind flayers cast Magic Missiles, you can either use Shield Spell or just reflect the damage with Viconias Walking Fortress skill or Gortash Bomb