Question: Changing a Battle Command to a single spell

The easiest way is probably to sacrifice Banon's Health command (which just casts Cure 2) or Leo's Shock command (which just casts Mega Hit). Just give them a different spell index and you're done.

If you want to replace mimic, you basically need to copy the code for one of those commands, change the spell index, then make mimic call that (either by copying it over the mimic code or copying it into free space and changing a pointer)