He was doing a rolling swing. Disregarding if that move is feasible anatomically, the pure weight behind the attack is heavy.
If you argue her strength grows with shorter distances, then her initial blocking (initial battle) with shields is unnecessary, she should've been able to block the attacks regardless of shields. (As in block with empty air, or with nothing)
You underestimate the body's neurological shock response upon....
Bro was swinging down, unless the shock caused his muscle to spasm like an electric shock.... The weight of the attack should still go down. It would be like an executioner with an axe swinging to chop a head, and you blow his heart out with a pistol. I would wager a good chance the axe will still go down.
Page 11, you can see he seems to swing straight up and let the scyth swing down by gravity and strength. He isn't swinging sideways. Even if you cut off his strength, the scyth is still falling downwards. Unless this is equivalent to electric shock response were all his muscles spasm to hold the scyth in place. Also that scyth is coming down in less than a second with that force (initial height, weight of scyth itself and propulsion by his swing)
Mechanics aside. I still think crushing his brain was the proper solution.