For all his might and power, Firkraag is kind of lame. I'll be honest and admit that some research took place before the battle proper, but it's too easy to kill him. I didn't even hide or anything.
You'll need: 1x Fireshield (Red), 1x Protection from Fire, 3x Skull Traps (preferably in a Sequencer, if only to speed things up), lots of Magic Missiles (you probably won't need it, but it's nice to have), Melf's, maybe a Spell Immunity (tbh you won't need this either), 1-2x Mordenkainen's Sword (position your summons close to him and keep a little distance - the AI will target his Dispell on Mordenkainen's), 3x Lower Resistances, and 1x Feeblemind (the killer spell).
Just walk over to him, fire up your protections, summons the Swords, place them accordingly, and initiate the fight with a Lower Resistance. Firkraag will waste his Dispell on the Sword, keep on firing those Lower Resistances (you'll need all three of them, just to be sure). He might throw fireballs (or fire breath, whatever) at you, but you're perfectly safe with the Fireshield and whatnot.
Ready for the super lame part? Hit him with Feeblemind. Game over. Have fun pummeling the guy with everything you've got.