Graviga has been changed to be non-elemental, which mean Kefka won't do any dark damage besides Bioga.
Bring tons of 法力值 recovery items, you will need them.
"Puppeteer's Wheel" will charm 1 unit, and it's mechanically similar to stop. The unit won't be able to do anything for a couple of turns. You can either wait it out or kill and raise the unit.
"Kefka is skipping!" doesn't remove blind.
Noctis could be considered for his 生命值/法力值 Recovery with Cover as well as Comeback just in case your main healer is KO'd by one of Kefka's non-elemental super attacks.
The easiest way to kill Kefka is to exploit the matching elemental debuff against him. Equipping an elemental weapon (if the ability is element-less), and waiting it out for the specific elemental debuff to appear, and then one turn kill him with powerful abilities. It'll deal 59x damage because of the debuff.
A single powerful unit (900+ ATK) will deal a devastating damage in one turn, but will not kill him outright. You will need another unit to help with the damage.
If you have Setzer you can kill him easily with fixed damage from Double Dice. Equip him with any elemental weapon, without regard to ATK. Double Dice damage are random and you have 40% chance to hit 55555 or 77777. Multiplied by 59, that's an instant KO. If you didn't hit the 55555 or 77777, close the game and turn it on again, you are back at the beginning of the round, and use the other Dice attack. It will give another value.
Even 5555 becomes an instant KO if you Dual Wield with Poach if single-elemental; dual-elemental 5555 with Man-Eater from two sources and two Poach also works
If you are dual wielding, make sure to use only one element, otherwise the weakness is halved.
Setzer is an exce积分ion, he can use two different elements while dual wielding to cover more elements.
If you are trying to kill/overkill Kefka with 极限技, that 极限技 will have to deal damage. Any 极限技 damage after it's dead will count even if it's not technically the killing hit. It's recommended that the Esper gauge generator below are the one doing the 极限技 mission by having a non-elemental weapon and non-elemental 极限技 damage. Do not use elemental 极限技 unless it's a matching element with your finishing strike.
Using Void Vessel while he's debuffed with dark will deal 118k damage each. You can hold a maximum of 5 Void Vessels.
Void Vessel damage can be increased with magic killer effects. If you are using Cloud of Darkness, it'll deal 75% more damage, 206.5k. Similarly, Exdeath and Kefka has magic killer against humans, though in lesser value.
Double evocation is the hardest part of the battle.
Fully resisted attack will not drop any Esper orbs, so if you are using units to kill him with elemental attack, they will have to take a backseat for most of the fight.
Use one or two dedicated unit to generate Esper gauge. It's important that the unit will have to equip a non-elemental weapon. Give them immunity to blindness.
There are several ways to farm Esper orbs in this fight:
Counter: Snow is the best unit for this. Kefka uses a lot of normal attacks, giving you more chance to counter. Cecil with Black Belt can also be used, and he covers side healing as well.
Garnet or Rydia 极限技: Since they are currently max 5星 as of writing, you will have to equip them with a lot of defensive gears. O积分ionally you can add an entruster like Ling or Bartz.
Chains: Chains increases chance to drop 极限技/Esper orbs. Use two Dual Wield Setzers with Prismatic Flash to rapidly accumulate 极限技 and Esper gauge with considerably cheap 17 法力值 cost. Other chainers combination (ie Chizuru's Phantom Shadow) should work but exhaust more 法力值.