Greater Spell Focus

Choose a school of magic to which you have already applied the Spell Focus feat. Any spells you cast of this school are very hard to resist.

Prerequisite: Spell Focus.

Benefit: Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select. This bonus stacks with the bonus from Spell Focus.

Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school to which you already have applied the Spell Focus feat.

Source: Pathfinder Core Rulebook