I wrote before that a critical hit dealt to an elemental immune target would use a fixed attack power value (25 for the SNES version, 21 in the GBA version)...
Well, this didn't get tested in FF4A, but I suspect this will still be true.
For Edge, he's a special case.
If both hands are equipped and one of them exposes the elemental immunity, it should use the standard fixed attack power value that I've mentioned before.
If one hand is equipped, and it exposes the elemental immunity, the formula works more like this...
Attack Power = 25 + weapon power/2
Regular formulas still apply...
This is readily testable in JCE's hack.
We have EVEN MORE themes now, but some old ones are gone...