I just fought Draconis a couple of times to see if my suggestions were any good. EDIT: Harm and Call Lightning don't seem to be effective; when they hit him, they don't do anything. /EDIT But by using the strategies I normally use, I was able to defeat him every time. The second battle was much smoother than the first because it took a little while to hit my stride, so to speak, so I'll describe the second battle.
My PC is a Cleric/Ranger. I also had Minsc, Sarevok, Keldorn, Imoen, and Jan in the party. They all have 4-5 million experience points.
To start the fight, my Cleric/Ranger cast Iron Skins, Remove Fear, and Death Ward and walked up to Draconis by herself while he was in human form. (I wanted to keep everyone else out of the way.) When the fight started, Draconis cast Horrid Wilting, Time Stop, and Symbol: Fear. When those were out of the way, I brought in the rest of my party. Because Draconis had cast Mislead, Keldorn counteracted it with True Sight. When Draconis was visible again, Jan cast Breach. Then Keldorn, Sarevok, and my Cleric/Ranger pounded away. Draconis cast Sunfire once or twice, but fortunately, each of my nearby characters had some sort of fire protection (rings, armor, helms, etc.), and my other characters were out of range (like I had planned). I ignored the invisible stalkers that Draconis had summoned. When Draconis was badly injured, he switched to his dragon form.
At that point, I pulled everyone back except for my Cleric/Ranger so that she could do the "Rocky Balboa" routine. I had her cast Iron Skins again and had her read a Scroll of Protection Against Acid (to protect against Draconis's breath weapon). Draconis wasn't able to hurt her very much, but he dispelled my character's protection spells a couple of times, so I had to re-cast them. I couldn't hurt Draconis at that point, and he kept turning invisible and summoning invisible stalkers, so I ignored him and killed invisible stalkers just to pass the time. (I was waiting for his Spell Immunity: Abjuration to wear off.) I killed about a dozen invisible stalkers, and Minsc and Sarevok actually levelled up during the fight.
After about 25 rounds of that, I brought in Keldorn again with True Sight and had Jan cast Breach again. Minsc cast Call Lightning and targeted one of the invisible stalkers (on my first few attempts, I wasn't able to target Draconis, and the spell was cancelled). I let the invisible stalkers surround one of my characters so that Imoen could kill all of them with a Death spell, and they were gone. (The next few lightning bolts were supposed to hit Draconis, since he was the only remaining enemy in the area of effect.) Then I turned my attention to Draconis.
Jan and Imoen both cast Lower Resistance, which presumably wiped out all of Draconis's magic resistance. I kept Jan and Imoen away from Draconis so they wouldn't be attacked, and they never got hit. They cast Chain Lightning a couple of times, and it was pretty effective. I kept an eye on Draconis's protection spells and countered them whenever it was necessary. (Stoneskins and Protection from Magic Weapons = Breach, Minor Spell Turning = Ruby Ray of Reversal, etc.). I didn't let Keldorn cast Dispel Magic because I'm not sure whether it would remove the effects of Lower Resistance. (The Wizard spell Dispel Magic is not supposed to remove those effects, but I can't remember if the Inquisitor's form of Dispel Magic is any different.) When Draconis was in dragon form, I had to cast Breach a total of three times, and I cast Ruby Ray twice (so that the lightning bolts would hit him). Keldorn cast True Sight whenever it was necessary. In the meantime, Minsc, Sarevok (who was also protected by a Scroll of Protection Against Acid), Keldorn, and my Cleric/Ranger "tag-teamed" Draconis. I used Critical Strike a couple of times. (I should have used it more, but I was trying to see what works and what doesn't.)
Draconis healed himself once during the fight, and he cast Remove Magic on my party members several times. He has a way of counteracting just about everything. If I'm interpreting what I see with Infinity Explorer correctly, Draconis is not affected by Sarevok's Deathbringer assault. He appears to be immune to Harm, Call Lightning, and Energy Blades, as well. I tried to use Harm, but usually when I tried to hit him with it, he instantly put up Stoneskins. When I tried to target him with Call Lightning, he instantly cast Invisibility and the spell was cancelled. When I did manage to hit him with Harm, there was no effect, and even when I saw a lightning bolt hit him, he didn't suffer any damage. I'm not sure if that had anything to do with the protection spells that he had cast, but I didn't see him take any damage from Energy Blades, either. (So much for my earlier ideas.) I didn't bother using spells like Mislead, Project Image, Invisibility, etc., but if I had, I figure he might have cast Dispel Illusion or True Seeing automatically. His "reflexes" are something else.
I don't feel like I did anything all that fancy. But I used the space bar quite frequently to pause the game, and I turned off AI for this particular fight so that I could put my characters exactly where I wanted them and make them do exactly what I wanted them to do. If I had been more aggressive, the fight probably would have been shorter, but I took my time and got the job done.
I think the main thing that throws everybody off is the Spell Immunity: Abjuration. I don't know what else to do except wait for it to expire. Draconis also deals out quite a bit of damage if you let him. (He killed Sarevok, Minsc, and Imoen in my first battle with him today, and he injured my Cleric/Ranger badly, so I had to use a Rod of Resurrection and regroup.) I think the best way to limit the damage is to keep your party members separated instead of keeping them all in one place where they can be attacked all at once. That's why I recommend "tag teaming". Careful use of True Sight and Breach also made things go smoothly. In hindsight, I'd have my Cleric/Ranger memorize Heal spells instead of Harm.
I don't mean to brag, but the fight was much easier than I remembered it. That's probably because Draconis made a really strong impression on me the first time I fought him, and I will always regard him as a unique and worthy opponent. But now that I have so much practice and know how to handle him, it's a lot easier. I'm also still a bit numb from playing Icewind Dale 2, so I didn't get scared or feel much of anything at all, really, and that probably made things easier (but not as much fun).
One thing I really like about the Baldur's Gate series is that the villains are so cool. Instead of adhering to strict D&D rules, they have all sorts of special abilities that make them great opponents.
I hope this helps. Good luck!