The core character build is a Bard/Rogue/Fighter that uses two green items - Helmet of Arcane Acuity [1] and Band of the Mystic Scoundrel [2] - to create highly accurate (i.e. high DC) spells to keep him under control. As you'll see in the video, he'll be permanently controlled with 2 spells: Hypnotic Pattern and Tasha's Hideous Laughter.
Notes:
- Gameplay is in Tactician (max difficulty)
- There's a minor hiccup where one of the Cambions on the left didn't get hypnotised, and that really threw me off
Character Build:
- Fighter 2 / Bard 7 (College of Swords) / Rogue 3 (Assassin)
- Abilities: 8 str / 16 dex / 14 con / 10 int / 12 wis / 14 cha (in reality I had 17 dex with the following bonuses: +1 from Hag's hair and +2 from Mirror of Loss; giving 22 dex in total)
- Feats: Sharpshooter
- Core items: Helmet of Arcane Acuity and Band of the Mystic Scoundrel
- Other items: A good bow (I used Gontr Mael [3]), and for armour I had Graceful Cloth [4] for the +2 dex but feel free to use other armours if you want more defense; other slots are pretty much flexible.
- Important Bard spells to take: Vicious Mockery, Tasha's Hideous Laughter, Hypnotic Pattern, and Confusion
How it works:
- Arcane Acuity gives a +1 spell attack roll / DC for each weapon attack (capped at +7)
- Band of the Mystic Scoundrel allows you to cast an Illusion or Enchantment spell with a bonus action after a weapon attack
- Bard (College of Swords) has Slashing Flourish (Range) that uses 1 Bardic Inspiration charge to do 2 hits for a weapon attack
- Together with Haste (buff from another party member) and Action Surge (Fighter), we'll get 3 actions in total, and that's 6 weapon attacks. With Slashing Flourish and 4 Bardic Inspirations we'd be able to make 10 hits in the first turn, all with advantage (Assassin), effectively maxing out the Arcane Acuity bonus (+7)
- With Band of the Mystic Scoundrel, we can use the bonus action to cast Hypnotic Pattern or Tasha's Hideous Laughter with high accuracy on the same turn as they are Illusion and Enchantment spells. This is why Raphael couldn't do a thing in the whole fight.
Links:
[1]: bg3.wiki/wiki/Helmet_of_Arcane_Acuity
[2]: bg3.wiki/wiki/Band_of_the_Mystic_Scoundrel
[3]: bg3.wiki/wiki/Gontr_Mael
[4]: bg3.wiki/wiki/The_Graceful_Cloth