This is theoretical. Going through a book... really.
To display an overkill flag, you'd have to create an overkill value in Game_Battler, like this:
attr_accessor :overkill # overkill flag
Then, in Game_Battler's initialize... set it to false.
Well, you'd probably have to edit (or alias/replace) the
attack_effect definition in Game_Battler 3. Your target would be on or around line 71, after a random number formula (# Dispersion) routine.
After that, compare that final value (self.damage) to the battler's health (self.hp) to get your value. If the value is... an overkill value (yer choice... percent of health... whatever)... then you turn the 'overkill' flag I mentioned to true.
Now, you have to go into Sprite_Battler, and around line 108 (#Damage), get the damage statement to accept the 'overkill' flag to show, like it does the 'critical' flag. Then turn it off with an @battler.overkill = false statement.
Now, adding an extra percentage to your exp. Dunno.
Probably have to do this in Scene_Battle.... really rough outline...
Set @overkill_total to false
If (somewhere) @battler.overkill == true... then @overkill_total = true
.....(doesn't matter if only 1 overkill or 5 overkill kills)....
If Victory... if @overkill_total == true...
...exp = exp + (exp / 10)
...@overkill_total = false (just in case)
end
Oh, and to get the 'OverKill' message to show up on the screen, you have to edit the
SPRITE MODULE itself. It has the 'Damage' statement that is invoked IN Sprite_Battler that I mentioned above. An example of the 'damage' statement can be found in the Remodeled Damage Display script... though it replaces the text displays with images.
Probable redesign would start with...
def damage(value, critical, overkill)
...so when you call damage, you'd include both the critical AND overkill flags.
Technically, you could have a "CRITICAL OVERKILL" message if both flags were true. :D
Again, this is pretty rough... and though it means actually CHANGING the system, these changes would probably work with RTAB as well... as long as no other RTAB Add-On script overwrites it.