Ironskin

Transmutation [earth]
Level: Alchemist* 2, Antipaladin* 2, Artificer * 2, Bloodrager * 2, Cleric */Oracle * 2, Druid * 2, Hunter * 2, Paladin * 2, Ranger * 2, Warlock * 2
Components: V, S, DF/M (a pinch of forge soot)
Casting Time: 1 Standard Action
Range: personal
Target: you
Duration: 1 minute/level (D; see text)
Saving Throw: -
Spell Resistance: -
  Your skin hardens and takes on the color and texture of rough iron. You gain a +4 Enhancement Bonus to your existing Natural Armor Bonus (if you do not have a Natural Armor Bonus, you are considered to have an effective Natural Armor Bonus of +0). This Enhancement Bonus increases by 1 for every 4 Caster Levels above 3rd, to a maximum of +7 at 15th level. While you're under the effects of this spell, if an opponent confirms a critical hit or sneak attack against you with a physical weapon (not a spell or magical effect), you can dismiss this spell to negate the critical hit or sneak attack and treat it is as a normal hit. Dismissing the spell in this way is not an action, but you must be conscious and aware of the attack to do so.