I'm gonna ignore the Ur-Priest, because it is such an obviously idiotic PrC. It takes PrC to a whle new level of idiocy. The designer should be dragged through the streets, tarred, and run out of town. And the editor incuding in the book it was published in should suffer the same fate. Flavourwise, ruleswise, it's just stupid. It is the epitome of everything wrong in 3e.
That said, never underestimate the defensive staying power of a divine spellcaster.
A divine spellcaster avoids combat until the others have slaughtered each other, and the last spellcater has used up all their offensive spell repitoire.
A common mistake is to feel like you have force an action. Don't. Let them make the mistakes.
Via fairly simple means, a high level cleric can render himself untargetable for a surprisingly long amount of time. Play the waiting game. The wizard's invisibility wears off after a while. Fog Cloud, Darkness, Obscuring Mist, & Meld into stone, are your friends. Those arcane spellcasters will likely be focusing on the offensive, and unlikely to have considered counters to your defences. Unable to carry backup items (scrolls, wands, potions), just makes their cause even more precarious.
IMX most players are far too impatient in these kind of exercises. Why chase you, when there is another perfectly visible character (detect invisibility, anyone?) within range?
In a freeforall, even if you do have the most powerful character, making yourself obviously the most powerful, is a very dangerous tactic, as the survivors are more likely to gang up against you. Even a Monk/Ur-priest has difficulty keeping up with the actions of two equal-level opponents.