How about instead of the final shot actually KILLING the boss, it puts him into a state similar to Gears of War's "bleeding out" state?

That way, in order to deliver the Finishing Blow, which would have to be done at point-blank range, you actually have to have control of the boss's immediate vicinity? Maybe it could even be an assassination animation so that it's less likely that some random guy from the other team could sprint in and Yoink the Victory Points without someone there noticing and Showstopping him first?

Personally, I find both the "51% damage" solution and the "last MSG bullet" solution irritating and discouraging.

Maybe something like this proposal could get around those problems?