Firkraag Himself - Baldur's Gate 2 Side Quest
The Firkraag Himself side quest begins when speaking to Firkraag inside the Windspear Hills Dungeon. Really all you need to do in order to resolve this quest is defeat Firkraag, that's what I am going to talk about on this page. Firkraag can be quite a handful depending on what level your characters are and how much you know about the game.
If you're a beginner then I have a better guide for you than what's on this page, check out my BG2 Beginner's Guide for tons of helpful information. This guide is going to assume you know the basics of self buffing and things like that which I cover in my Beginner's Guide. You should also check the wiki for their strategy on Firkraag.
What worked best for me is buffing up my entire party and summoning 3 Animate Dead along with 2 Conjure Animals. You can summon 5 Animate Dead if you want, it's probably better to do all 5 Animate Dead because they stay active long enough to buff up with all the same buffs you use on your party.
On your melee the most important buff is typically Improved Haste since that doubles your amount of attacks per round. Remove Fear/Resist Fear is going to be the second most important buff to use on your entire party since dragons like to fear. If you notice this buff getting dispelled during the fight it's also the most important one to refresh. Fear lasts forever in this game and you're not going to survive your whole party getting hit with it against Firkraag.
As the wiki says, when it's time to engage Firkraag you'll want to do so with 1 party member. Within (usually) the first 5 seconds of the fight Firkraag will use Dispel Magic on the person who started the fight - you'll be able to tell because almost all of the buffs on his portrait will be gone instantly. At this point you'll want to engage Firkraag on everyone who is melee and also move your spellcasters in close enough to hit him - the fight begins in earnest now.
Keep in mind that depending on what difficulty you're playing on this fight will be dramatically different. I play Core Rules unmodded and all of the information I am about to give is centered around that. When the fight starts the first thing I do with my casters is debuff Firkraag. I use Greater Malison and Lower Resistance on my Wizard(s) and Doom on my Priest to make Firkraag more vulnerable.
After the fight goes on for about 10 seconds I will cast Breach and Ruby Ray of Reversal or Pierce Magic on Firkraag to strip him of many magical protections. I typically wait about 10 seconds after the fight starts to give Firkraag time to buff himself up. If you're playing on a modded version of BG2 that makes fights harder then you'll probably want to open the fight with stripping these buffs.
Below is a list of all the debuffs I typically use against Firkraag during the fight.
Debuffs to use on Firkraag:
Greater Malison: I recommend you throw this at Firkraag at the start of the battle as it'll decrease his Saving Throws and make him easier to hit
Doom: This spell stacks with Greater Malison and will decrease THAC0 and Saving Throws
Lower Resistance: Using this on Firkraag will help a lot of your magical spells to land against him. Lower Resistance stacks too which is worth noting, you can use multiple against him to lower his resistance significantly.
Breach: This is the lowest level spell protection removal that can work against Firkraag and it won't remove everything. However, it's better than nothing.
Ruby Ray of Reversal: One of the higher level spells you can cast that will strip Firkraag of many magical defenses.
Pierce Magic: Much like Ruby Ray you'll want to cast this on Firkraag to strip magical protections from him.
Once you've debuffed Firkraag as much as you can what I typically do is launch a few Magic Missiles at him to see how many land. If over half of my Magic Missiles land then I consider that good enough and start using more powerful spells against him. He's a dragon (obviously) so don't expect every spell you cast at him to land - or deal full damage - but that's ok.
This is a borderline cheese method but something you can do is use Spell Sequencer or Spell Trigger and pick multiple Magic Missile spells to put into them. Once you've debuffed Firkraag enough you can unload all of your Sequencers on him to deal a decent amount of damage.
Another cheese strategy that you can do is use the Set Traps Thief skill or the Time Stop Trap HLA that you can unlock on Thieves. You can place these before the battle starts and they'll trigger once Firkraag becomes hostile to you. There are dozen more strategies that you can use against Firkraag but this covers most of the well known ones.