After Kefka absorbed the magic of the Warring Triad, he became a god. His power was vast enough to destroy towns and even devastate the world. Yet, a ragtag collection of misfits defeated him. They’re nothing special. They have access to Espers, but their powers are nowhere near Kefka’s. So, how could he lose? It may have something to do with Kefka’s philosophy of nihilism.
Life… dreams… hope… Where do they come from? And where do they go? Such meaningless things…I’ll destroy them all!
According to the philosophy of nihilism, nothing matters. Nihilism is the rejection of all religious and moral principles, often in the belief that life is meaningless. Kefka could have constantly cast Heartless Angel, reducing our HP to 1, to continue to toy with our party. Every time we cast Curaga, he could cast Revenger to stop all magic effects. Then afterward, he could cast Fallen One on one player at a time, taking them out of the fight forever. And just to end it, he could finish us with Havoc Wing with a laugh. That horrible laugh.
But he thankfully doesn’t do any of that. In fact, given his power and position in life, he seems relatively disinterested in continuing the battle. When he was a low-level mage, he took pleasure in coming up with creative ideas to torture people. Here, nothing. Perhaps this was just a distraction to our party. While fighting our entire party, he might have been firing the Light of Judgement in various directions of the World of Ruin. This gives us more incentive to defeat Kefka, before he reduces everything to ashes.
Also, he’s not exactly sane. Maybe even after a year of godhood, he hasn’t learned to actually manage his powers efficiently. Nihilism, in a sense, is a philosophy lucid enough where only the insane could seriously adopt. If life, dreams, hopes, and nothing else matters, why fight? Why is Kefka fighting? Logically, there’s no reason. And he knows it. But as insane as Kefka is, he’s inconsistent. He does defend himself, perhaps only as a mean to continue his destruction spree.

But then something doesn’t seem to make sense. If he finds the time to destroy, he must values destruction. If so, why does he live with the hopes to cause more destruction? Yet, at some point during the fight, he realized something. If he destroys everything, he must die as well. But how can he end his life? Taking this thought to its logical conclusion, he lets our party kill him. He achieved his goal, for a while, and decided it meant nothing. So he decides to destroy as much as he can. In his sick, twisted reasoning, the world our heroes go back will have nothing. Then, once he dies, they will die out of lack of resources and despair.
But The Returners destroy Kefka before he can accomplish this plan, proving their victory over nihilism.