Ritual spells designed to kill by some math theory
No, this is not what you don't study enough and your teacher kills you with a calculator. This would refer to a spell of ritual, which is designed to kill by things like the number of past targets, or a geometric arrangement of kill sites. It's not just for killing enemies though, but also the key to break spells (though in this case, the other end usually involves massive death). For example, suppose you have an enemy that is impossible to kill because of various MacGuffin sustaining its life. You could hunt down 6 or 7 of these, destroying them, in which case the enemy would suddenly be weak to your attacks. A subtrope of Ritual Magic and Geometric Magic.