how to kill a lich in a couple easy steps:
1. enter his line of sight, he fires of a spell trigger consisting of spells like Mislead, Fireshield, Stoneskin or Protection from Magical Weapons. cast True Sight to dispell the Mislead
2. after that lich starts casting Time Stop. just back off out of his line of sight and once the time stop starts he'll just sit there doing nothing
3. after Time Stop expires use a spell to remove spell protections (Pierce Shield, Pierce Magic, Ruby Ray, Warding Whip, Spellstrike...)
4. use Breach to dispell his Fireshield and Stoneskin
5. whack him once with the Mace of Disruption +1/+2. most of the time liches in SoA fail their saves and just die. if he saves, just whack him again, I haven't seen a lich make a succesful save twice in a row
you can follow a similar strategy with any other mage in the game. you just don't whack him with Mace of Disruption but with your most powerful weapons.
what you might also try to do, if you have a mage wearing the Robe of Vecna, have him cast Melf's Acid Arrow at the mage when you first spot him, before his protections go up. that will most likely disrutp his spellcasting for a few rounds. you can even use a (minor) spell sequencer with two or three Acid Arrows, that will almost be enough to kill the mage outright. or you can put Flame Arrows into a sequencer, two or three of them will almost certainly kill the mage. the important thing is to fire off those Acid or Flame Arrows before he can put up his protections.
also, use weapons that do some elemental damage (Flail of Ages, Frostreaver, Firetooth...) as that goes through Stoneskin. a great weapon for disrupting mages is the Short Bow of Gesen since it's electrical damage goes through almost any protection.
also, you can get a thief, have him wear the Cloak of Non-detection (or use the spell), hide in shadows and try backstabing the mage.
hope you find some of this useful...