If you have the Cloak of Mirroring or the Shield of Balduran, then the Beholders don't stand a chance. But if you don't have those items, you can expect a pretty tough fight.
Even if you cast a lot of protection spells on your party, the Beholders can dispel them, so don't count on 100% protection. Still, you can try to avoid Hold spells, poison damage, death spells, and petrification if you have the right scrolls and spells. You also need to watch out for Imprisonment, which the Elder Orbs can cast. If you don't have a Berserker or a Scroll of Protection from Magic, about the only thing you can do is try to kill them without getting close to them.
You need to disrupt the Beholders' spellcasting any way you can, and you need to inflict as much damage as you can as quickly as possible. To accomplish the former, I like to use Spook, Glitterdust, and Emotion (preceded by Doom and/or Greater Malison). To inflict lots of damage quickly, I like to use Fireball/Sunfire, Skull Trap, and Glyph of Warding. Hasted Fighters with Potions of Giant Strength can certainly help, as well, but watch out for Remove Magic, Death Spell, Flesh to Stone, and Hold spells. Summoned monsters might help, but they won't do you any good if you plan to use area effect spells like Fireball, which will hurt your own helpers.
The first room is probably the hardest, because you walk right into a Death Tyrant and a Beholder without any chance to prepare for battle or to position yourself strategically. The other groups of Beholders are bigger, but they might be easier to deal with if you use stealth and scout the area before you attack them. You can also let the Drow war parties soften them up.
I just took a look at the Beholder strategies page, and aside from the cheese (for example, using an item that makes you invincible or casting Cloudkill off the screen to kill monsters who just sit there taking damage), there was one major error: Beholders are immune to Web/Hold/Entangle/Grease/etc. If you use those spells, all you will do is waste a round, endanger your own party, and make the Beholders angry.
When you use the Cloak of Mirroring, you can see what works against Beholders and what doesn't when their own spells get bounced back to them. For example, they are susceptible to Petrification, Charm/Domination, and I think the Death spell and maybe Cause Serious Wounds, but they are not affected by Hold or Slow. From what I can tell, they don't have any special immunities and they don't use protection spells, so there's no need to cast Breach, Pierce Magic, Lower Resistance, etc. (Except for Elder Orbs, which do have 50% magic resistance.) And they are always susceptible to physical damage if you can last long enough against them to hit them.
Good luck!