[F. Gottwald]

as i said above, i don´t have a standard way of taking out mages as such... you just have to pay attention at what exactly they cast on themselves. some usual things:

area spells - skull trap is a staple unless they have globe of invulnerability, if they do, just use higher-level area spells.
weapons with elemental damage if they rely on stoneskin
keep a few normal weapons with you in case they cast protection from magical weapons
soak up their spells until they have used up everything dangerous (usually with someone polymorphed into a mustard jelly, possibly center a death fog right on it)
most really unpleasant wizards are liches so anti-undead measures work well.
of course, anything that kills them outright (backstab, medium/high lvl offensive spells) or causes continuous damage (bleeding, poison, acid) before they get their defenses up is also helpful.