Undead can be a huge pain to deal with the first time you face them, as they revive 3 turns after you KO them. There are two main forms the undead come in:
Tombstone Form: The undead turn into this form when they reach 0 HP. Cast Burial, Exorcise, or (almost) any revival skill (the White Monk's Revive does not work on them) on them. Phoenix Down will also work. Any other skill which states it can banish or kill Undead will also work. The chance to hit in this form is 100%.
Normal Form: The undead are in this form when their HP is greater than 0. Casting burial or exorcise on them has a small chance of permanently killing them, regardless of how much health they have left. Any other skill which states it can banish or kill Undead will also work. The typical chance to hit in this form is 35 to 45%.
Other skills such as the Paladin's Sanctify, the Hunter's Oust, and the Bard's Requiem work regardless of their state.
You can increase the accuracy of these abilities with the Green Mage's Tranq, the Archer's (for Humes) or Fusiler's (for Moogles) Passive ability Concentrate, or casting Stop (Time Mage) or Ring Toss (Juggler) on them.
The Alchemist's Transmute stands out by itself, because it can transform HP Critical enemies into items. This is useful for the undead, because you won't get any items off them otherwise. It works in both forms as "HP 0" still counts as HP Critical.
It should also be noted that any mission that requires you to "defeat all foes!" will be completed if all foes either removed from the field (by KO or an ability), petrified, have the Toad status, or in tombstone form.
Some other things to take note of when fighting undead are:
- They will revive into their normal form after being a tombstone for three turns at full HP (they auto cast Doom on themselves after being KOed)
- They can be healed by Dark-element spells, weapons, and abilities
- They can be damaged by any holy based healing spell (cura, cura, et cetera)
- They can be damaged by certain healing items, such as a Phoenix Down or Hi-Potion
The Final Fantasy wiki was used as a source for some of this information.