Okaaaaaaay... then you'll have to do it with your good old tactics. There are a lot of posts on killing beholders: read them. Also, if you know what spells they cast you can prepare yourself for those specific spells. I don't really remember them (I think Gauths cast Cause serious Wounds, Lightning Bolt, Hold Person etc. and Beholders have an antimagic ray, Flesh to stone, also cause serious wounds) but if you tried to fight them as often as you say then you'll probably know what spells they cast. Incidentally, I think that on beholders, weapons work better than magic.

Btw. I should advise you to download the Bonus Cd anyway: it doesn't have an install file but you simply put it in your override folder and then two new merchants appear in the Copper Coronet & Adventurers Market. They have lots of cool and very useful items which you might be able to buy later on in the game when you have more money
Try:
www.tgeweb.com/ironworks/baldursgate2/
or
http://dspace.dial.pipex.com/m.james/

I did this before I added the extra merchants, and it's hard but completely doable.

I cast mass invisibility, and got everyone in position before beginning the attack. It was still bloody, but I killed them with only a loss or two on my side which is easy to fix.

When I did this mission, I killed the gauths by summoning a lot of creatures, then cast pixie dust (so that everybody is invisible) then go and surround the gauths. After you are in position, do whatever you want, it will be easy.

All my characters at this point was level 9, except my cleric/mage

Another thing that works really well:

Stand at the very edge of the bridge, just out of the range of the beholders' vision, and use wands of fire(ball) and/or cloudkill on them. As they cannot see you they won't attack. It's even better if you do this before answering the bridge's questions, this way they can't reach you. You may have to use farsight to be able to center the cloudkill on them (if the area is still 'black' you can't target any spells in there). You can also cast these spells on them with an improved invisible wizard

Incidentally: in your first post you say: warriors, fire, traps
I think traps can never work on beholders as they LEVITATE above the ground

Im searching now for aomething i cant remember (a staff piece or something like this) and, after passing the bridge´s questions, there are 3 gauths and some shadows. Shadows are easy to beat but the gauths hurt really bad and I they seem to have lots of life. My party is level 9-10, except me, sorceror level 11. What should I do to kill them ?? Ive tried anything: warriors, fire, traps...

THANX

Well if you have the bonus CD (the items from this cd are also downloadable, e.g. from The Ironworks Page amongst other sites) you can buy the Shield of Balduran from Deirdre: this reflects beholder's rays back to them. I used this shield on Viconia and she singlehandedly killed all the Beholders in the Unseeing Eye quest. Another items that reflects magic damage back is the Cloak which you get in the Sahuagin city.

Thanks Melusine but i havent the CD neither the money (in the game) to buy any shield. And i havent been to this city (in fact i have only been to part of Ankathla and Nalias castle), so Ill have to kill them other way.

Try summoning fire elementals, they work wonderous against gauths.

Yes and if you have cleric/mages: summon undead (preferably Skeleton Warriors)
They are virtually immune to magic!