Kefka is the final boss of Final Fantasy VI, fought for the fifth and final time at the end of the game. Having become the God of Magic itself, Kefka faces the party after declaring his desire to destroy the bonds of life and end all existence. The final phase of the song, "Dancing Mad", plays during the battle.
The battle is fought in several stages. Kefka always uses Heartless Angel on his first turn, reducing the party's HP to 1, so it is advised to heal with Curaga immediately after or, even better, a Megalixir. He will most often use Firaga, Blizzaga and Thundaga, but will also use his special, a powerful physical attack called Havoc Wing, that has high power and can easily KO a character. Kefka will also use Trine, inflicting Blind and Silence on the party. The player should use Remedies to heal.
The second stage begins once Kefka's HP has been reduced to 32,640. Kefka will now begin using his ultimate attack, Forsaken. The attack begins with Kefka stating "The end draws near..." ("The end comes...beyond chaos." in the SNES/PlayStation versions), followed by a head representing Kefka's laughing expression appearing in front of him. His turn ends, and the background begins to shake. On his next turn, Kefka unleashes the attack, doing high non-elemental, unblockable magic damage to the entire party. Despite the preparation time, base power and animation, Forsaken actually doesn't inflict very much damage. This likely stems from the fact Forsaken doesn't ignore split damage or magic defense. By comparison, Ultima, which does ignore magic defense and has only two thirds of the spell power of Forsaken does significantly more damage. This would make Forsaken very powerful indeed if it did ignore defense and/or split damage.
After Forsaken's first use, Kefka will begin using two turns for Havoc Wing, Trine or Vengeance, which eliminates any and all positive status changes from the party (meaning it will not remove Blind or Silence from Trine). When Kefka's HP hits 30,080, he will begin Countering virtually any and all attacks with Hyperdrive, a powerful magic attack that ignores defense and will likely kill its target. When Kefka's HP drops to 10,240, his counterattack changes to Ultima. Ultima is very likely to kill the party, so it is advised the player only attack when Kefka is preparing Forsaken, as he will not counter when charging it. The final stage begins when Kefka has only 7680 HP left; he will begin alternating between Forsaken and Meteor, and after using Forsaken once in this stage will no longer need to charge it up. He will continue to counter whenever not charging up Forsaken.
Since Kefka is the final boss, the player should use their strongest spells and attacks. Having a character with high magic power on standby to cast Curaga on the party is not a bad idea in case the player is caught off-guard, and buffing spells like Haste and Shell are also useful. As long as the player keeps their HP high and does not trigger Kefka's Ultima counterattack, he should fall in time.
A cheap yet painlessly easy way to defeat Kefka is to have a character equipped with two Ultima Weapons (possible if a second is stolen from Rest) and a Master's Scroll. As the Ultima Weapons ignore defense, at a high enough level the character can kill Kefka in one attack - they will attack him eight times, each blow only needs to do a minimum of 8000 damage to defeat Kefka in one turn. Using the Bahamut Esper to boost the character's HP is a good way to get the HP needed for this type of damage output at a relatively low level. This will also work with Locke's Valiant Knife if he has high max HP and is on low health. Setzer's Fixed Dice also work with this strategy - with Setzer at only Level 50, a roll of triple fives or triple sixes will do 9999 damage, and triple fours will still do high damage. Thus, with enough lucky rolls, Setzer can kill Kefka in two or three attacks regardless of level or attack strength - using the Quick spell helps.