God Kefka had the power of the statues, which is all the magic in the universe of FF6 (with the exception of the Espers). Hell, even regular Kefka could kill 8 Espers at once.

Kuja managed to take a hit from Bahamut with barely a scratch. When he got the power of Trance, he was able to cause a lot of damage to Terra (he probably didn't destroy it completely; maybe it was similar to Lavos in Chrono Trigger). And he was able to kill the party by putting nearly all of his energy into one final Ultima.

Neo X-Death had to power of the Void, the greatest power in FF5 (except for maybe the crystals). He was able to easily destroy towns and villages with the Void, and in the final battle, he said he could destroy all existence and during his Grand Cross attack, it showed a message saying something like "the universe is in peril."