What type of enchantment do you need to hit Firkraag? I always assumed it was +3 weapons (+3 weapons can hit just about anything if I recall), but I see some people talking online about using arrows, and you don't get +3 arrows until ToB.
There was one site where someone said to use ice arrows, and those arrows don't have any enchantments aside from doing additional cold damage.
Fighting Firkraag, as with any dragon, is all about forgetting your own buffs, which he will remove, and de-buffing him. The most important spell (spells are a must) to memorise or have scrolls for is Lower Resistance -- you MUST get his resistances down. Then you can Greater Malison him and Melf's Meteor the hell out of him. After you've removed most of his protections, most magical weapons and arrows will work. Oh, and *never* underestimate the power of Chromatic Orb!
Actually, Lower Resistance and Greater Malison do not combo with Melf's Minute Meteors. The meteors are not affected by magic resistance or saving throws; therefore those spells provide no benefit if you are using the meteors to attack.
Chromatic Orb, meanwhile, ignores magic resistance if your level is high enough for it to kill. However, the target gets a +6 bonus on its saving throw, so without penalizing its saving throw (Greater Malison and Doom are useful here), it might not have any chance to work. (A wild surge could cause it to ignore saving throws, but that's only a 1% chance per surge, so I don't consider that strategy viable outside of segmented speedruns (or a TAS if one is ever made).)
What you can do, on the other hand, is use Breach (if the enemy casts Death Ward), Lower Resistance, Greater Malison, and Doom, and then follow up with Finger of Death or another save or die. There's no rule requiring bosses to be immune to instant death, and it turns out that dragons typically aren't! (If they were, they wouldn't be casting Death Ward.)