Yeah, I first edited the code where the attacker is in back row, i.e. when a character attack.. Now I edited the right code and it work, a monster attack previously doing 34 damage in back row now does about 52 damages.
Code:
hirom
org $C20D11
jsr free_space
nop
org $C26469
free_space:
rep #$20 ; 16-bit acc
lda $F0 ; load damage
lsr a ; divide by 2
lsr a ; divide by 2
sta $10 ; save in temp RAM
lda $F0 ; load damage
sec
sbc $10 ; damage - 25%
sta $F0 ; save as damage
sep #$20 ; 8-bit acc
rts