[Taqwus]

Well, the fountains likely have spells to duel each other with, so it's not inconceivable that one wins. They might also summon monsters to help...

If neither kills the other before the time limit, the attacker presumably explodes at the "all attackers who haven't retreated instantly die" mark.

I wonder how viable an all-offense 'Ginsu the Damned' approach (e.g. Nataraja casting Curse and Astral Weapon, then using a ridiculous number of attacks to maim the enemy into a blind, crippled, feebleminded mute) would do; it'd be readily countered by regen, I think, but would perhaps be amusing anyway.

Different tack from a hypothetical 'Zzzz' approach (e.g. Ettin Mandragora w/ E3, RoWizardy, stockpile of Earth gems, casting Petrify and then attacking with those nasty sleep vines), or perhaps an absurdly extreme 'Tim "More Power' Taylor' approach going for pure damage.