Well, to begin with, you'll need all the useful spells and a really good equipment. When a character is hit by an attack, it's a guaranteed kill. Therefore evasion, magic evasion and Life3 is really important. Thanks to "Mirage" you can avoid getting hit by most phsycical attacks and Celes Runic can be used to absorb magic... even though I didn't find it useful in the Final Battle against Kefka since most of his attacks are "special attacks" and not normal spells, thus they cannot be absorbed... not to mention the fact that you can't use Magic yourself when Runic is activated. It's pretty useful in some other battles, though.
Anyways... surviving IS the really hard part... in almost any battle.
Terra can do some pretty decent dammage with a double casted Ultima. Gembox is easy to get if you've got the Moogle Charm and in order to have enough MP to actually cast Ultima, you'll also need that item which makes all spells only cost 1 MP. So... Terra's your main killing-machine. However... most of the time you'll be busy reviving and stuff... so therefore this battle will most likely take much more than 1 hour... not to mention the fact that you'll probably fail countless times before you actually manage to beat him.
Oh, yeah... and if I remember correctly, Necronopticous actually defeated Kefka by using "Rasp" until Kefka didn't have any MP left, making him unable to attack. I've never tried to do that myself, though. So I don't know whenever that's more or less effective than just attacking his HP.![]()