First off: Welcome to the Board! [img]graemlins/thewave.gif[/img] Second: it's not stupid or so that you did this, but these posts (concerning Baldur's Gate 2) should be in the forums that they belong in. No big deal; you know now, don't ya!
I can understand your frustration, because Nizi (I don't even want to begin to try typing its name correctly!!) is the toughest dragon I think in the whole of Baldur's Gate 2.
Probably the worst thing he does, is using his Insect Plague, which slowly kills you characters. The best thing you can do therefore is try and avoid getting hit by that spell (easiest thing to do that, of course, is killing it before he can use that spell
Dragon tactics also apply for this particular dragon, so Lower Magic Resistance (to use spells such as Magic Missile, Chromatic Orb etc.) and Breach (to dispel Stone Skin and other spell protections) are definitely a must if you want to defeat the overgrown lizard.
If it still doesn't work, you might try the search engine for tactics concerning either 'Nizi', or 'the black dragon'. Good luck!