You don't need more than 8PS to oneshot people on a regular basis, if you turn into you swings, hit the head and make sure your opponent is running in your swing too.
But the real power of the flamberge is sneaking up on people to finish them off. As you are slow, when you reach the combat it has already begun and many people don't have all their health. If they have ~40 HP left, you oneshot them fairly often.
You should avoid getting in duels, and concentrate on ganking people, like a pikeman.