From what I remember of FF6 classic and the one video I could find of atma weapon dying, the boss death animation is a combination of a color tint, fade out, and shaking. Also the flash of white, which can also be achieved with a color tint effect.
Here's another dissolve type effect I made an example of before, based on the Undertale style. construct.net/en/forum/construct-3/how-do-i-8/dust-effect-monster-dies-157891