Enemy bigger than 128x128?

Here's an idea/question, what about making it into 2 enemies, but only 1 is attack able. Is there a way to make them both "light up" when the one is attacked and have them both die when the one dies?

You cannot have two 128x128 enemies in one formation. Open the formation editor in FF3usME and you can try different mold combination available.

As for the brief white flashing when the monster attacks, you can't make two monsters flashing when one attacks, at least not in any way I know.

You can however make any other monster die in the formation when a certain monster dies. Just enter this after the end of first wave of attack.
Note that this example make all the monsters die like a boss:

Code:

FC 12 00 00    ; If following monster is/are dead:
F5 0C 01 FF    ; Monsters #1, #2, #3, #4, #5, #6 are killed, dies like a boss