The rules of D&D are built to allow people to have a faster-paced tabletop game with set, simple, and defined numbers tied to more complex situations. There are surely ways to do nearly anything in the DnD world, but asking for a Fireball [a spell you get at level 5-6 (if I remember correctly, I haven't played a Sorc/Wiz in years) mind you] to render a beholder obsolete is absolutely idiocy. For the same reason that a level 20 pixy can get hit with a 40 lb hammer and survive easily, you can't just explode all of a Beholder's eyes with a single spell unless that spell happens to kill the beholder or specifically have an effect that would do such a thing. Cleverness is nice, and it can benefit you GREATLY in D&D, but mistaking "I do fireball! It's AoE! Beholders have lots of eyes! AoE hits all eyes!!" for cleverness would be foolish.