for the finishing move, what about haveing it scale based on either health percentage or some less fiddly method of that idea?

so for example:
100-80% health: 1d6 damage/class level
79-60% health: 1d8/class level
59-40% Health: 1d8/class level, enemy shaken for class level/2 rounds
39-20% health: 1d8/class level, enemy knocked prone for class level/2 rounds
19-0% health: death effect (could maybe even get away with death effect with no save at that health level)

make it a once per encounter ability, and your set.
possibly at higher levels allow an AOE version that scales down the dice by one step or something.

if the percentage is a bit fiddly, you could work it based on enemy hit dice in relation to your own but that seems even more so.