From the Super Mario Wiki, the Mario encyclopedia
Instant death attacks are a class of special attacks from Super Mario RPG: Legend of the Seven Stars. The term refers to any special attack that can one-hit down a party member, regardless of either the party member's or the enemy's stats (such as Hit Points, attack, and defense). Attacks of this variety include Magnum and Shaker. Instant death attacks can be blunted by pressing at the right time during attack, allowing the party member to keep either one HP or all HP. If the timing was perfect, the party member keeps all HP as though nothing happened, but if it is slightly off, the party member still survives with a single Hit Point. Certain accessories, such as the Jinx Belt, Safety Ring and the Quartz Charm, can prevent instant death attacks from having any effect.
Instant death attacks