What are some ways to deal with someone who is paranoid and untethered to reality?

This is a person I have no choice but to deal with for the next ten years or so. It is a person who is completely paranoid and as a result is constantly on the offensive trying to make problems for me. This person is not above purposely hurting people that mean a lot to me just to get to me. I am at my wits end in how to deal with it and since I have to for the foreseeable future, I am hoping to get some advice from people with some kind of experience with similar situations. I have tried to deal with the situation rationally but to no avail. Attempting to share my concerns is also not an option.