Originally I understood this phrase as "I will rebuke this man, cut him in two with a great difference (in score)" but have not been able to sufficiently break down my understanding of ひと断ちにする to my own satisfaction.
I originally read ひと断ちにする as similar to 一太刀にする (although 一太刀 actually taking にする seems to be rare according to exact google search) but that may not be correct.
To my understanding 断つ means to sever something, so I think 断ち is its noun form, making ひと断ち one slice by my reasoning, so I think literally he's saying that he will take him in one slice by a great difference.
Am I on the right track or am I missing something?
EDIT: I should have made this more clear. 佐為 is talking about figuratively slicing someone in a game of go. It's not literal.