When you say “most legal issues,” it is a little hard to understand as there are many issues that could be connected to a supervisor’s work. I will assume that you are referring to legal issues that are connected to negligent supervision and negligent retention since you mention those things specifically.
Negligent supervision occurs when a supervisor does not adequately pay attention to or control their worker’s actions. For example, let us imagine that I work in the same work space as my supervisor and my coworkers harass me sexually. If my supervisor does not pay attention to what they are doing even though (s)he is present, I can argue that (s)he is guilty of negligent supervision. As another example, imagine that my supervisor does not train a new worker and does not stick around the work space to see if that new worker is able to handle power equipment that we use in our job. That could be negligent supervision because the supervisor has not adequately monitored the coworker to see if that person is capable of working without endangering me.
Negligent retention occurs when a supervisor knows that someone in our firm is likely to hurt someone and does not do anything about it. For example, let us assume that I am a nonwhite person and my supervisor finds out that a coworker has a history of violence against nonwhites. The supervisor does nothing with this information and instead lets the two of us work together without supervision. If the coworker attacks me, I could accuse my supervisor of negligent retention.
Two main characteristics that a supervisor should have and use to avoid these sorts of legal issues are attention to detail and conscientiousness. A supervisor who has these characteristics will be careful to know as much as possible about what is going on in the workplace. The supervisor will not simply take the easy way out and assume that things are well. If there are issues, the supervisor with these characteristics will not hope that they resolve themselves. Instead, the supervisor will take steps to ensure that they are fixed. Such a supervisor will be more likely to avoid most of the legal issues that can arise from negligence.