Anti-Sanctuary - reverses all penalties and bonuses. The beholder's minions then attack the target preferentially.
Mislead - targeted on a melee bruiser minion that PCs would gang up on if they saw him coming.
Summon Monster X - give the PCs something else to do while the beholder watches from the sidelines.
Bless / Bane - because drenching the PCs in molasses is a great way to make yourself look like a tougher enemy.
Liquify - as Petrification but the PC turns into a puddle of water. Can be reversed by anything that removes status conditions - like the Restoration spells.
An eye ray that causes a status condition might make for a unique fight. Deafened might not bother most PC parties (much), but how about Blinded?

Be aware that a PC with Flight can follow your beholder, should you choose to retreat. Ground-pounders usually can't do that.