A level 13-15 party of three PCs taking on a balor? Sounds crazy, but it's possible... just!
Everything hinges on you setting the time and place for the encounter, though! When fighting a superior enemy the element of surprise is your strongest weapon. If the balor knows you're coming, your chances are very slim indeed! If you can somehow catch him flatfooted so that everyone in the party gets a surprise attack, that will be your best hope of success.
Also, get some help! Hire some seasoned mercanaries and a wizard or two; guarantee them ressurection after the fight if you have to. If you get the chance to set the time for the battle, you only need to hire your helpers for a few minutes time!
Appeal to any good-aligned churches or other organizations for help; anything they can contribute would be useful: Clerics, paladins, warriors, magic or simply cash.
Buff everyone up as much as you possibly can (don't forget fire-protection!) and Teleport in. Having a Freedom of Movement spell on each character is also a good idea, as it will keep you from getting entangled by the balors whip.
If you can't get a wizard to come with you, consider buying/borrowing a ring of spell storing and get a wizard to cast some spells into it. Quickened True Strike is a great aid for melee experts with the power attack feat! Of course, Heal is also a good choice, but I'd go for buffs my self; try to make the battle as brief as possible! Don't bother putting any offensive spells into a ring of spell storing; you'd probably need to roll a 20 to beat the balors SR...
Make sure everyone has weapons that can overcome the demons DR! The monk can pull this of through feats from BoED, if you use it. Also, get Holy or Outsider Bane weapons, if you possibly can! If nothing else, at least coat your weapons (or hands, in the case of the monk) with holy water.
You also need to have some way of dealing with the balors spell-like abilities. One possibility is having a spell caster along that does nothing but stand around with a readied Greater Dispel Magic, prepared to counter anything the balor attempts to cast. Dimensional Block can also be useful, to stop it from Teleporting away.
One of it's many spell-like abilities is Greater Dispel Magic - at will! This could blow away your buffs rather sooner than you'd like, but will automatically be countered if you bring along somebody who does nothing but cast Greater Dispel Magic.
Another option is of course to use anti-magic field to stop it from using spell-like abilities (and teleporting away), but that would probably hurt your PCs more than the balor. After all, the demon (probably
) has far better stats than the PCs without magical aid.
Of course, all of these measures and precausions indicates that the PCs knows exactly what they're getting into. :\ If no-one in the party has sufficient Knowledge Planes, roleplay them gathhering information from every available source! After all, they know they're going up against a vastly superior foe and if they don't know too much about it, it makes sence for them to try to lear as much as possible. "Know Thy Enemy" has always been a wise motto.
Hope some of this helps. I know it's largely dependent on your playing a fairly high-magic campaign; if you're playing a low-magic campaign I honestly don't think you stand a chance. Unless of course your DM has nerfed the balor to better suit the campaign. If that's the case there's really not much I can say, not knowing how the balor's been changed...
Good luck anyway, and let us know how it went afterwards!
![:]](http://www.enworld.org/forum/images/smilies/devious.png "Devious :]")
Sorry to be the bearer of bad news...
