Twisted Innards

Transmutation
Level: Alchemist* 2, Artificer * 3, Magus * 3, Sorcerer */Wizard * 3, Warlock * 3
Components: V, S, M (a cocoon tied with string)
Casting Time: 1 Standard Action
Range: personal
Target: you
Duration: 1 minute/level
Saving Throw: -
Spell Resistance: -
  For the duration of this spell, your vital organs writhe, shift, and move about, making it difficult to strike you in a vulnerable area. While this spell is in effect, critical hits and sneak attacks against you have a 25% chance of failing to inflict any additional damage-though you still take the normal damage from the attack. At 7th level, the chance to ignore additional damage increases to 50%, while at 13th level the chance increases to 75%.