We talked about epic characters in skeptic's thread, then we all got hit by the digital nuke. I think I even highlighted that it's fun to see how many beholders you can take out!
It sounds like your group found out...the hard way!
One thing that probably needs to be right near the top of the epic player's guide:
Losing players is probably fairly common. Keep in mind that you can rez players the way a low level cleric can heal them. So, it's ok if your players succeed, and half go down. They'll be right back. I agree with skeptic and frankthedm as well, however, that the defenses used by a typical 21st level party should be very sufficient to deal with beholders. Rogues and Monk types should have really good touch AC's to force missed attacks, clerics (and others) will have SR to avoid attacks, Paladins, Clerics, and Fighters should be almost untouchable on Fort saves (14 base, 2 base stat, 3 stat buff, 5 res bonus - this gives you a base +24). Ready access to cover and concealment effects also provide another layer of available defenses. Lastly, as Frankthedm points out, with only one "type" of villian, it's easy to counter their abilities and made quick work of them with "anti-magic shell".
The only other suggestion I've got is to invest in contingency effects.