spoiler alert!
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no offense, but I found Draconis to be a big bad sissy. I moped the floor with him on my first try, then cleaned out Abzigal's lair and Abzigal himself without resting inbetween. then I went on to defeat Balthazar and finish the final challenge in the Pocket Plane. all this without resting and never had to reload. and since I was feeling over-powerfull, I thought, 'heck, let's give it a try' and I just entered the Throne of Bhaal. I managed to defeat Melissan, then killed the first guardian, defeated Melissan again and defeated the second guardian. but then I have completely run out of useful spells and the third fight with Melissan got really, really, REALLY hard without having any Summon and Simulacrum spells left. so I just reloaded to the pocket plain, rested and went in to finish the job... she wasn't even a challenge, the final fight lasted about ten seconds.
but I'm kinda off topic here...
back to Draconis - just summon as many creatures as you can, and absolutely summon a Planetar or a Deva. be prepared to summon another one in-battle if the first one get's killed. also get any possible Elementals (Greater Elemental Summon works the best of course
, Golems (you do have the Golem Manual, don't you?), Berserker Warriors (if you have the Iron Horn of Valhalla from SoA), cast Simulacrum on all your mages... if you have Vhailor's Helm (another SoA item) you can use it to have one of your fighters cast Simulacrum - get an extra lvl 20+ fighter. also don't forget your fighters' special abilities (whirlwind attack, smite, deathstrike...).
then once he turns into a dragon, be sure to cast a Breach spell once in a while, as he like to use Stoneskin, also use Lower Resistance and Greater Malison to increase the effectivnes of your magic attacks. Abi-Dalzim's Horrid Wilting does crazy amounts of damage to him, if you get his resistance down.
hope this stuff helps, remember it's also aplicable in any other fight against magic using opponents...