When a wizard casts Lower Resistance upon a target creature, he lower's the Magic Resistance of the creature by 10% + 1% per level of the wizard.
|“|| Lower Resistance|
When cast upon a target creature, this spell will lower the Magic Resistance of this creature by 10% + 1% per level of the caster. There is no Saving Throw and the target's Magic Resistance, if any, does not affect this spell. For example, if a creature has 60% Magic Resistance and this spell is cast on it by a 15th-level Mage, then the target's Magic Resistance would be lowered by 25% automatically. This effect is cumulative for each casting of this spell: If Lower Resistance was cast upon the same creature again, the creature's Magic Resistance would be 60% - 25% (initial casting) - 25% (current casting) which would leave the creature with 10% Magic Resistance. This spell will last until its duration expires and cannot be dispelled, even by Dispel Magic.
In some cases (like when facing dragons) lower level parties are actually better off using Magic Resistance (Spell), a priest spell, on enemies. Likewise, it allows no saving throw and actually sets MR to (caster level)*2%. Thus a level 12 cleric will set the dragon's MR to 24%, while a wizard of the same level will lower its MR by -22%. Now, if that dragon has 60% MR it's obvious that 24%<(60%-22%) and thus Magic Resistance is the clear winner.