Instant Death is an effect that removes units from battle, playing their death animation.
Interactions with Damage Mitigation and KO Resistance
Instant Death ignores all forms of damage reduction and KO resistance. As such, there is currently no way to prevent Instant Death from KO'ing.
Elemental Synergy: Prism
Upon using a Dual Brave Burst with the Prism synergy, the caster will KO itself. Additionally, the unit cannot be revived until the current wave has been cleared.
Elemental Synergy: Blast
Upon using a Dual Brave Burst with the Blast synergy, all units not affected by Doom revive and become invincible for a certain amount of turns. Upon buff expiration, all units instantly become KO'd, resulting in a Game Over in most situations. Additionally, units KO'd from the effects of Blast cannot be revived until the current wave has been cleared.
Upon a skill that caused Instant Death to a unit, it is possible to survive the attack if the attack frames overlap with another AoE or ST attack frames that is occurring simultaneously, provided the unit has Guaranteed KO Resistance from Extra Skill, Elgif, or SP Enhancements.