I was looking around Deling and found the Cerberus fight event (ID 685, found in field.fs)
Looking at monster0 and talk, I've found this code:
LBL 52
UCOFF
PSHM_B VAR344
PSHN_L 128
CAL OR
POPM_B VAR344
PSHN_L 13
SETBATTLEMUSIC
PSHN_L 118
PSHN_L 0
BATTLE
BATTLERESULT
PSHI_L TEMP0
PSHN_L 4
CAL EQ
JPF LABEL0
HIDE
THROUGHON
TALKOFF
PSHN_L 12
MENUNAME
LABEL0
PSHN_L 5
SETBATTLEMUSIC
UCON
RET 8
Looking at PSHN_L and SETBATTLEMUSIC in sequence, I noticed the PSHN_L 13 above and PSHN_L 5 below, showing that the event change the music to 13 Force Your Way and then gone back to 05 Don't Be Afraid.
Now the challenge is how to put it in random battles... Or if possible, to just create a new NPC and put him to enter a battle like the Ultimecia's bosses in her castle.
EDIT:
The PSHN_L 118 means the Cerberus battle, but how would I put the code to recognize this? Just putting it below SETBATTLEMUSIC? Or theres something else I'm not seeing?