Now the only reason Colbert would win, is because there are enough fans of the guy who were there when he got his head shaved, and a Beholder does not get its hit points back that quickly all someone would need to do is bring a strand of hair to a cleric and they could cast a raise dead spell and then the adventure can continue. Now seeing that the Beholder could would wipe out the many incarnations of Colbert he would be reduced to level one pretty soon, but the shear amount of incarnations of Colbert would at some point overwhelm the Beholder, by shear pile of body causing plague or by statues cluttering up the Beholders den or any number of things, but given enough time as long as there is enough hair and enough fanatics to bring him back he would eventually win.
But ONLY because the odds are that the dungon master is a huge jerk who likes nothing better than to murder awesome/popular characters in the cheapest way possible.
Colbert would win if the fight was fair, but after Colbert rolled his 137th natural 20 in a row, the DM would say "Oh, too bad! The beholder gets him with a shot with the disintigration ray. Nope, no save! Colbert now has -80 HP, his body is vaporized and doesn't get any kind of saving through and if you don't like it, tough!"