Weaken is a debuff introduced with the Elf Princess Ally. It causes any enemy that is hit by the attack to receive 10% additional damage from all player sources for the remainder of the battle. The effect of weaken can stack with itself, so far up to 3 times from an ally and once from the Jester Call Bunnies skill, resulting in a 35% increase to damage dealt.
The effect persists throughout the battle until either the player or the enemy is slain. Bonus damage inflicted is a multiplicative bonus to the base damage inflicted by both the player and his current ally. For instance, if a player inflicts 5000 base damage, after 10% weaken, the player will inflict 5500 damage. After 20% weaken, the player will then inflict 6050 damage, and at 30% weaken the player will inflict 6655 damage. Critical hits, as usual, will double the base damage being inflicted.
The Copy ally skill is not directly enhanced by Weaken; it simply inflicts 25% of the damage the player's skills inflict regardless of how much is inflicted. Since Weaken increases the base damage inflicted by the player's skills, Copy also sees a damage increase due to that.