Demon Slice is a Limit Break for Seifer, only used during the battle with just Seifer at Galbadia Garden. It inflicts non-elemental physical damage to one target, and incidentally Seifer likes to target Squall the most often with this attack.
demon is "an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell."A
Oni is the kun'yomi rendering of the Chinese interpretation of an ogre called the "guǐ" (gu-ay). They are a kind of yōkai from Japanese folklore, variously translated as demons, devils, ogres or trolls. In a Chinese context it may refer to a ghost as well.