AFAIK, it is not possible to execute the part of that battle script while Rydia is dead. The assumption is that Rydia is alive and hidden in the battle (although this battle enforces that Rydia is hidden initially in this battle)... so when Golbez or "Rydia" tries to cast a fake "Mist Dragon", it fails miserably.
Edit:
The second issue you face is the "unhide Rydia" spell. The simple fix for that is simply removing that out of Golbez's battle script.
It is suggested that you make "Rydia"/G0lbez cast a spell to kill off the Shadow Dragon (I dunno what you would choose, but anything that kills a 1HP monster is easy enough to figure out).
We have EVEN MORE themes now, but some old ones are gone...