Very nicely written! Not many people bother to write about their defeats.
Use Aerie to cast Protection from Fire and/or Resist Fire on everybody. 100%+ Resistance to Fire is nice, but 75%+ is sufficient too. This is to handle his Dragon's Breath. Potions of Fire Resistance will serve the same purpose as well.
Firkragg likes to cast Chaos as well, so cast Chaotic Commands on Minsc and Valgar. You have Lilarcor, so you are already protected. Potions of Clarity will serve the same purpose as well.
Use Aerie and Jan to conjure up some summons prior to talking to Firkragg. Try to use your best summons, then cast Resist Fear on everybody. Protection from Evil 10' Radius is an excellent spell to cast beforehand as well.
During the fight, summons stay in front of Firkragg while your melee party members stay at the back of Firkragg. Have Aerie to cast Chant and Bless on them, then either Jan or Aerie cast Haste on them, then have her and Jan cast Lower Resistance on Firkragg followed by Greater Malison and Doom. You may use offensive spells like Polymorph on him now.
Have fun, and good luck.