Here's a novel way to kill a dragon. The catch is, you can only do it once (i think) because I only found one item of this type in the game, in the Underdark(there may be more). I tried it on Lord Jierdan Firkraag:
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Get your mage to cast Improved Invisibility on himself. Then have him stand to the side of the dragon while the rest of the group engage the dragon in melee. They will take some major damage but it will only be for a short time...one round as a matter of fact unless you are unlucky. Then get the mage to use a Scroll of Imprisonment on the dragon. As your mage has Improved Invisibility chances are the dragon won't notice him until its too late.

The downside? All the dragon's treasure vanishes with him but you do get the 64000 XP. Hey, I didn't say it was perfect...

If you're going to take this route you can make things a lot easier for you and your party and not even engage the dragon in combat. Inprisonment will work on non-hostile creatures as well as hostile. You can simply walk up to the dragon, imprison it and be done.

Drink up.

-Tipsy

I was going to mention that but being a good (as well as arrogant) character I thought I would at least let him have a go at me. Plus I wanted to hear the dialogue and make him swallow his words. If I was evil then that would be another story...

There are two other ways to easily kill a dragon and get the treasure.
1) Use lots of traps. Yoshimo putting 6 traps dopwn, sleeping adn putting 6 more should kill Firkragg. The downside is I do not belive you get the xp for teh kill. You do however get the treasure.
2) Cast an area spell such as Cloudkill close to where he is but when he is still off the screen. Repeat as needed.

You can cast freedom and bring him back then imprisonment again

I only ever found one Imprisonment scroll, and I used it on Neeber.