This is how I understand the suggestion:

Combat is done exactly as it has always been.
Combat is displayed differently.

Sometimes, not being hit will be displayed as a successful defensive action.
Sometimes (a random chance determined by your offense and target defense) the killing blow will be (displayed as) something a bit more impressive than the regular slash or smash. Maybe if the killing blow knocks the victim down to -1/10 of its (max vitality + defense * 2). (A 100 HP monster with 50 defense would get it for being knocked down to -20 on the final blow, and then there might be only a 20% chance that it happens still)

Conditions for special kill:
Monster HP (after death): Less than or equal to - ( MaxHP + 2 * Defense ) / 10
AND: Random value from 1 to 5 = 5.

Of course, this won't happen if the victim (I mean "monster") is killed by a special move.