お前も私も一人にせず、且つお前に 少しの 罰を 与える 方法を考えてみた。
In this phrase I've some doubt about "一人にせず". Using google, I can translate it as "not alone". But I read "せず" is a negative form of "する” and indicate that one action took place without or in the absence of another action. But... It seems senseless.
The sentence makes perfect sense.
Structure: "I have thought about a method that would achieve "Primary Goal" while satisfying "Condition/Prerequisite"."
Primary Goal = お前に少しの罰を与える = "giving you a little punishment"
Condition/Prerequisite = お前も私も一人にせず = "without having to leave you or myself alone"
一人にする means "leave (smb.) alone". So 一人にせず could mean "without leaving alone" or "not remaining alone".
To suggest a good translation for your quote we need more context.
