I personally would use a Beholder in a large cavern, and have him use stalacites for cover, as well as going behind several prepared escape routes if the need called for it, like tunnels or caves, and once in the tunnel/cave have it collapse the entrance with it's Disintegrate ray. I would mostly use hit and run, because the Beholders low Fortitude save and hitpoints make it highly unlikely it will survive if even one barrage of attacks is released on it.
The party's best bet is to try and immobilize or slow the Beholder in this case, because if it manages to remain at a good distance and is able to escape at a moments notice, it's highly unlikely the party will be able to survive unscathed. Focus fire from it's rays may be likely, making life tough for one particular party member.
I'd rank the classes like so, if I was choosing a team to go up against a Beholder;
1. Monk. Not even a contest, fast movement, great abilities, SR, and the best saves of all, make the Monk by far the best. The Monk can close very quickly with a Beholder, and will likely avoid any negative effects from it's rays. And once he's close, his stun will likely take the Beholder out. The Beholder could even mistake a non armored Monk for a spellcaster and waste his rays on him.
2. Paladin. Second best saves, great damage dealing abilities, and the added bonus of his other abilities make the Paladin a great class to have along for a Beholder hunt. He, like the Monk, has the easiest time surviving the Beholders many rays, though he would have a harder time closing the gap. I'd recommend either flying equipment or a bow.
3. Druid. Good Fortitude and Will saves, making them mostly safe from the Beholders attacks. The Druid can, once he maneuvers outside the Anti-Magic Cone, do some serious damage to the Beholder either with spells or in melee, likely killing or at least severely wounding it. Higher hitpoints mean even the Inflicting eyes aren't going to do much damage.
4. Barbarian. Can close fast, great damage dealer, and does decently well against the most devestating effects from the Beholders rays, particularly when Raging. As the Paladin, flying equipment or a bow recommended. In fact I'd recommend that for any Fighter type.
5. Cleric. Has a few long range blast spells that will work against Beholders, like Flame Strike, which will severely wound if not kill the Beholder, and with a good Fortitude and Will save, is relatively safe from the Beholders rays. Like the Druid in regards to hitpoints.
6. Fighter. Give a Fighter a good bow and he could just be a Beholders worst enemy. Low Will save makes him a bit vulnerable to a few of the rays, but he should be relatively safe.
7. Ranger. Can't really use his stealth as well as against most enemies, but still a great damage dealer. Bad will like the Fighter, but that can't be helped.
8. Bard. Spells and songs probably won't do much, but can deal some damage with a decent BAB.
9. Sorcerer. Spells aren't much use against a smart Beholder, but if they can get out of the Anti-Magic Cone they will incinerate the Beholder. The real problem is the low Fortitude save, which will likely end up killing them if the Beholder focuses his eyes upon the Sorcerer. Better than the Wizard because he can use simple weapons.
10. Wizard. See above, but without simple weapons.
11. Rogue. Absolute worst character to go up against a Beholder, IMO. Beholders are much harder to sneak up on than most monsters, and because of the Anti-Magic Cone even spells like Improved Invisibility may not help. The Rogue can do great damage if he manages to sneak attack the Beholder, but he'd have to sneak up on him, and that's very difficult what with the Beholders high ranks in Spot and Listen. And did I mention Beholders cannot be flanked?
The worst of all, though, is the Rogue has low hitpoints, and a low Will AND Fortitude Save, making them dead meat in most cases if the Beholder should choose to focus it's eyes on him. Definitely not a class I'd recommend when going Beholder hunting.
