At last...
For my final encounter in Icewind Dale I went full autism and listed my useful defensive spells. I began to cast them all before facing the Luremaster, but because I casted Chant before a few others I was unable to continue casting my Cleric spells. So in I went, prepared to lose. Yet somehow, I took me only one serious attempt to win the fight: a first try, unbuffed, to see what I was up against. The second try, a total success. Only two dead party members: Yennefer (thief) who didn't have much health to begin with and I should have kept out of danger, and Shani (cleric) who was Chanting and thus had a reduced movement speed, even with Haste.
I prepared in advance to destroy the Dispell casting faggot by advancing my party to his position. As we decimated him, I had Triss (mage) run around the battlefield to disperse the enemies. Shani followed suit, alongside Jaskir (bard) and Yennefer (once her health got dangerously low). Meanwhile Geralt (ranger) and Zoltan (fighter) kept pummeling at the enemies. Soon enough I realize the fight was winnable and hoped nothing bad happened. Luckily the Luremaster never used his Lightning spell against me. After both Yennefer and Shani died, I grouped my party together so that Geralt protected Jaskir and Triss. I remembered Jaskir was no longer entirely useless thanks to his buffs, and gave him a bow and a few stacks of +2 arrows to help fight the heroes from a distance. Eventually they were all dead, and the Luremaster made his appearance. As you can see from the screenshots, my party was still hasted and it was just a matter of following the Luremaster around, disrupting his spells thanks to Zoltan and Jaskir's ranged attacks, while Geralt closed in for melee.
I can't believe I won the fight (in fact I just launched the game again to make sure the difficulty was set on Core rules). The Luremaster's speech is more powerful when I consider I lost to him many times in my previous run, and thus his words perfectly apply to my couple-of-months younger self. I will never forget you Luremaster.
I went back just to see if Hobart was still there. Appropiately, he was gone (I was hoping maybe I'd get a little speech or a "thank you" from him).