FF6 MDE Findings

First off, you won't be using a hex editor.

You need "FF6MDE" which stands for Final Fantasy VI Miscellaneous Data Editor. A lot of spells and abilities are tied to one and another, such as Shock and Megahit. Shock is Megahit, in a sense
Open FF6MDE and load 14-byte and look for "Megahit" under Miscellaneous. Open another copy of FF6MDE up and look for Disaster and copy those bytes onto Megahit.
Congratulations, you've now covered the basics of getting an animation working.

A lot of animation creation (or rather modifying since we're not importing our own graphics, just taking bits and pieces from other animations) is touch and feel. I can't teach you how to make the ideal animation for you because this isn't exact math where the outcome will always be the same. This is more similar to fishing, just a little more complex. Over time, you start understanding the bytes just by looking at them. You can tell what each byte is, or rather what that byte is used for.