[SixOfSpades]

When most Mages cast Protection from Magic Weapons, there are lots of ways around it:
1) Cast Breach on them
2) Cast Dispel or Remove Magic to try to remove enchantments on them
3) Incapacitate them (using spells like Emotion) for more than 4 rounds, so the PfMW will wear off
4) Use Normal (non-enchanted) weapons on them
5) Hit them with instakill spells, such as Finger of Death
6) Hit them with spells that deal elemental damage, thus bypassing their Stoneskin and hopefully disrupting their spellcasting

But when Liches cast Protection from Magic Weapons, however, you're in for a world of hurt, because:
1) Liches are immune to spells of Levels 1-5, hence they can't be Breached
2) Ditto for Dispel Magic and Remove Magic
3) Ditto for spells like Emotion (you might be able to pull off a Power Word: Stun, though)
4) Liches are immune to all Normal weapons
5) Liches are immune to Death magic
6) Most Liches, especially Weimer Liches, love casting spells like PfFire, PfCold, PfAcid, PfMagicEnergy, etc., granting them immunity to almost every kind of elemental damage--and of course that immunity can't be removed because of the Lich's damned spell immunities.

So, basically, once a Lich has cast PfMW, the only way to kill it is to wait until it runs out of PfMWs. Unless you're a high-level Cleric or Paladin of Good or Neutral alignment.