Your pain shall be two-fold.
Reduces the enemy's Defense.
Armor Break is a Sword Arts ability, used by Steiner. It costs 4 MP to use, and can be learned for 30 AP from the Mythril Sword. It has a 50% accuracy rate, but can be used repeatedly. It halves the target's defense.
X Armor Break is found on Auron's section of the Sphere Grid. It attacks and sets the target's Defense at 0, as well as nullifying "Armored", meaning that even non-Piercing weapons deal normal damage. The inherent armor property of armored enemies cuts physical damage to 1/3 if attack is not Piercing.
Auron's Banishing Blade Overdrive alongside Frag Grenade (whether by Using or Mixing) are the two only attacks in the game that have 100% chance of inflict Armor Break, in contrast to Armor Break skill that not always inflicts this effect.
X-2 Armor Break is available via the Warrior dressphere, but it's weaker than it was in Final Fantasy X, reducing the target's defense, but not completely eradicating it. It costs 4 MP to use and 30 AP to learn. While damaging the target, it also reduces the target's defense by 2 which be stacked up to 10. It can also be used via Lady Luck's Attack Reels by rolling a Cherry. It also appears in the form of Armor Eraser, usable by Mascot Rikku, which targets all enemies and reduces their Defense by 3. However, Armor Eraser does not inflict damage.
A technique that destroys the item equipped on the target's body.
Rend Armor is an ability learned by the Knight class for 400 JP. It has an instant cast time, an effect of 0, and its range depends on the equipped weapon's range. It attempts to destroy the target's equipped armor, and in the PSP version, will deal regular physical damage to monsters as well.
TA2 Rend Armor is an ability learned by the Master Monk for 350 AP from the Cypress Pole. It attempts to destroy the target's equipped armor and has a range of 1. In addition, it is also used by the Reaver, which in addition to trying to destroy the target's armor, will also inflict non-elemental physical damage.
4HoL The Scholar's Subdue ability halves all enemies' defense for three turns at the cost of two AP. Subdue is the second ability learned on the job. The Scholar's ultimate ability, Tame, reduces the enemies' defense to 1, but it only works for one turn.
TFFCC Armor Break will appear as an ability within the game.