Navigate to the class Game_Battler, line 683 in the standard RGSS2 class list.
Change this:
CODE
@mp_damage = damage
To this:
CODE
if self.mp < damage
@mp_damage = self.mp
damage -= self.mp
@hp_damage = damage
else
@mp_damage = damage
end
Then, any skill that has the "Damage to MP" option selected will damage MP. If the damage is greater than the target's remaining MP, it will subtract all the remaining MP and the remainder of the damage will come from HP.
I haven't tested this modification, though. Let me know what happens.