Undo Resistance( Fiendish Codex II: Tyrants of the Nine Hells, p. 85)
You learn to use cold iron weapons to unravel your opponent's magical defenses with a well-placed sneak attack.
One of the following: skirmish ability, sneak attack, sudden strike,
If you successfully deal sneak attack damage with a cold iron weapon, you reduce the target's spell resistance by 1 point per die of sneak attack damage. Multiple uses of this feat stack. Undo Resistance can reduce a target's spell resistance to 0, but not below 0. Spell resistance reduced in this manner returns to normal 1 hour after the last sneak attack.
You also gain this benefit with skirmish and sudden strike damage. Your DM might also allow you to use this feat with similar special abilities that grant you extra dice of damage against flat-footed or flanked opponents.