First things first, the beholder will be levitating out of reach of melee. It has a very high Intelligence and Wisdom, so it will recognize a spellcaster at the first sign of mumbling. Always assume it readies an action to turn its antimagic eye toward spellcasters, while the other eyes take out other foes. If it sees someone with a bow, it will keep its telekinesis eye ready for that person to deflect the arrows (it should be easy with that much force; it could even shoot them back at the archer). Then it will attempt to charm and scare off the archer.
Then the next round, once it has cancelled an attempted spell, it will fire off the death ray, petrification ray, and inflict wounds ray at the wizard, while continuing to try to charm or put to sleep warriors. The cleric it doesn't have to worry about as much, since they tend to lack ranged attack spells, and usually aren't big archers. If they are, it can always just block the arrows wi
th telekinesis again. If anyone starts flying, that person will be antimagicked to the floor.
With the thing's mad spot check, it should never be surprised, and so should always be considered to start combat with a readied action. It should only ever use its bite if it is being grappled by a wizard with an antimagic field or something similar.
They're CR 13 easily, if not higher. I personally would've given them fewer instant kill abilities and raised their HP and AC a bit to compensate. Maybe replace disintegrate with a fire beam (requiring a Reflex save), and the petrification beam with an offensive teleport spell to remove the person from combat without killing them.
Also, if you're going to have them fight a beholder, have it be underground, in a cave complex the beholder has designed. Using its disintegrate beam, it will cut numerous vertical shafts, with a few interconnecting horizontal shafts. Each shaft will have many narrow holes carved through it, so the lair resembles a block of Swiss cheese that someone cored out vertically. The eye tyrant can keep track of all the various passages, and can snipe from them if it sees a foe.