Cloud: I couldn't finish 'em. Looks like this's gonna get complicated.
The following tables are incomplete and require the specify if an attack sets HP to one, or does damage to lower HP to one. entries to be filled. If you wish, please examine the table and add anything missing. Remove this notice upon completion.
HP to one abilities reduce the target's HP to one regardless of their current HP. This is sometimes done by simply setting the targets to one HP, or at other times by inflicting damage to them equal to their max HP minus one. They are commonly used by final bosses and superbosses. One of the best known of these abilities is Heartless Angel, which has become the trademark attack of Sephiroth.
Megido Flame used by Omega Weapon will inflict 9,998 damage to the entire party, and thus the only way to survive is to have 9,999 HP, leaving the party with one HP after the attack hits.
Percent is a dummied spell accessible via Selphie'sSlotLimit Break. The attack reduces all enemies' HP to single digits, and uses Fire's spell animation. It cannot reduce an enemy's HP to 0 or deal more than 9999 damage.
Beatrix uses Stock Break and Climhazzard to reduce the party's HP to 1 when fought as a boss, and these abilities cannot be blocked or avoided. This ends the battle. When these abilities are used by the player's party, Stock Break and Climhazzard instead simply deal major non-elemental damage to all enemies.
Sight Unseeing has a small chance to reduce a susceptible enemy's HP to 1, while Souleater and Bonecrusher can potentially leave the user with 1 HP. Sap will also inevitably leave any victim with 1 HP if not cured.
In the Zodiac versions, Judge Ghis, when fought in the final Trial Mode stage, uses Elixirs on reversed party members, thus reducing their HP to 1.
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Fall damage from a high enough point will leave a player with no less than 1 HP, so long as they have not currently generated any enmity, in which said falls will prove fatal.
Attack
Target
Classification
Used By
Superbolide
Self
Reduces HP to 1 and grants Superbolide, rendering immune to damage for 10s
Reduces HP to 1 (is immediately followed by an Ultima AoE, requiring swift healing). Savage version inflicts Incurable, which prevents healing. The higher the target's HP at the time of being reduced to 1, the lower the duration of Incurable
Shango uses Divine Storm to reduce the party's HP to 1 when fought for the first time. This leads to the end of battle once he has used it. The same goes for Glacial Dance used by Cocytus. The battle are scripted events when these are used. When these abilities are later used by them during their real battle, they deal normal damage.
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Superbolide, rendering immune to damage for 10s
Gunbreaker
Blue Mage
Incurable, which prevents healing. The higher the target's HP at the time of being reduced to 1, the lower the duration of Incurable
Dynamic Fluid or 
Entropy.
Doom, requiring a full heal in 15s to avoid death
True Walking Dead, requiring a full heal in 15s to avoid death
Down for the Count when targeted with an emote