Metal Slash is a recurring ability in the franchise. It ensures it will damage a metal slime by dealing one extra point of damage. When the Machine monster family is not present, the skill will also deal 50% more damage to mechanical monsters.
In the remakes, Metal Slash is one of the special abilities that Psaro can learn. He learns it at level 40 (or earlier depending on his wisdom score).
This ability is learned with 13 skill points invested into the Sword skill.
If the enemy being attacked is not a metal slime enemy, then this ability is functionally equivalent to a normal attack. Otherwise, the enemy in question takes 1 or 2 damage (provided that no critical hit is dealt and the metal enemy does not evade the attack).