Um, Calaethis...most characters won't be able to cast "time stop" unless they A) do it from a scroll or B) have ToB installed.
What I would suggest is the following:
A) Summon "fodder" (summoned creatures) and occupy the dragon with those.
B) Cast "Wizard Eye" and make the dragon attack that while you whack at it from the backside
C) Put every kind of protection spell on you that you can, and haste all of your characters. For Firkraag, for instance, have everyone with Protection from Fire on.
D) Have your mages memorize all their abjuration spells possible (Ruby Ray, Magic Word, Spell Thrust, Pierce Magic, Pierce Shield (if you can cast it), Greater Malison (to reduce the dragon's saving throws)).
E) Have your characters lower the dragons' magical resistance by casting "Magic Resistance" on him...it actually lowers the resistance on a dragon! Then have a cleric or druid cast "Harm" on the dragon while the mages pommel him with "Magic Missile"...should take him out fairly quickly.
F. Summon fodder and cast "Otiluke's Resiliant Sphere" on them and send them up to the Dragon. The dragon will keep attacking the protected fodder while you take him out.
G. Cast "Feeblemind" on the dragon after lowering magic resistance and using Greater Malison. The dragon will sit there like an idiot (which he now is!
) and you can take him out quickly.
H. Have your thief surround the dragon with traps (this works on Firkraag and the Shadow Dragon, since they aren't initially hostile). Then lower resistance and Greater Malison the dragon, attack, and watch the traps do the rest.
Hopefully these strategies may help you. Good luck!
-Sazerac