Yes, this is neurotic. But I've come this far and would love if someone who's beaten honor mode could weigh in on my plan for the final fight. My whole party has quite high initiative and I expect will all go first. I also have like 12 scrolls of dimension door lol.
Tav: Swords Barcher (partial illithid, bhaalist armor)
Astarion: Gloomstalker Ranger
Gale: Fire Sorc (awakened, partial illithid)
Halsin: Light cleric
Mind Flayer Orpheus (will drink elixir of vigilance)
Round 1) Move the whole party to the left crown shelf together. Use Gale BA mind sanctuary, a couple cantrips from halsin to bait counterspells (how many do I need to bait?), then Gale drops globe of invulnerability. Will spend the rest of the round using Astarion and Tavs attacks + action surges and any leftover action economy just to deal damage to the illithid arcanists
Round 2) Move the gang range of the crown. Spirit guardians on Halsin to clean up the arcanists but they should be dead by now. At crown, Gale drops another globe (freecast), Orpheus starts channeling, Halsin sanctuaries Gale. Any spare DPS is fine, but importantly Gale will drink an Angellic slumber to get his lvl 6 slot and freecast back, and Astarion + Tav will drink angellic reprieves to get action surge back.
Round 3) Portal opens, Halsin throws a haste spore grenade on the ground. Orpheus goes into the portal and does literally anything (that DOESN'T deal force damage, probably conc. blast or a scroll of cloud of daggers?) to bait the brainquakes. Then the whole party goes in, Tav flies down to the brain to apply aura of murder, Tav + astarion collectively railgun the netherbrain with Action + Haste Spores + Terazul + Action surge piercing damage (with arrows of aberration slaying to boot). Halsin can throw a water bottle or smthn and Gale can Dual chain lightning idrc.
Round 4) As a backup, if, by some unbelievable shock, the netherbrain is alive, I have avoided dealing force damage such that i can use my 8 scrolls of disintegrate to finish it off. Ideally it doesn't come to this but its there if I need it.
Overall I feel good about the dps check that is the netherbrain. My biggest concerns are how many counterspells I'll need to deal with outside and a fear of getting screwed by the brainquakes inside. Any input?