I don't think one shot is going to do it.
Without a pre existing injury I think a prolonged beating could possibly be the end for a healthy fighter.
For example- imagine Kongo vs Al Turk, but it goes 3 rounds and ends like that fucked up joe Warren fight in bellator with loads of brutal shots but the guy doesn't go down right away. (This is why is happens in boxing, prolonged pumishment on a brain that has already received too much trauma from knockout type blows but the fighter was able/allowed to continue)