Damages with the difference between your max HP and current HP.
Minus Strike (サガク剣 or サガクけん, Sagaku-ken?, lit. Difference Sword), also known as Eye for an Eye (not to be confused with another ability of that name) is a recurring ability in the series. It deals non-elemental damage to one target equal to the difference in the user's Max HP and current HP.
IX Minus Strike is an ability usable by Steiner, learned for 35 AP from the Iron Sword, Rubber Helm, and Excalibur II. It deals non-elemental damage equal to the difference in Steiner's Max HP and current HP at the cost of 8 MP.
4HoL Eye for an Eye is the level 3 ability of the Dark Fencer class. At the cost of 3 AP, 2x Ruby, 6x Emerald, 6x Topaz, and 1x Amethyst, the user will deal damage to one target equal to the difference in their Max HP and current HP.