lone it
To go somewhere or do something alone; to be by oneself. A: "Do you want to come out with me and my friends to see a movie? B: "No thanks, I'm just going to lone it tonight." No one else wanted to leave the party, so I had to lone it back home.
lone gunman
A shooter that acts alone. This phrase is perhaps most often associated with Lee Harvey Oswald, the man who assassinated John F. Kennedy. I know you think the murders were perpetrated by a lone gunman, but I just can't believe that no one else helped him to carry out all this violence.
lone wolf
1. Someone who does not seek or like the company of others. Shannon should have known Brad would break her heart—he's a lone wolf who doesn't like getting close to anybody. A: "Does he have any friends at school?" B: "No, I think he's a lone wolf."
2. A terrorist who acts alone or independently of a larger group when carrying out an attack. The attack was carried out by another lone wolf who had no affiliation with terror groups but was inspired by radical postings online.
(lone) voice in the wilderness
One who expresses an unpopular opinion or idea. She felt like a voice in the wilderness as she tried to warn others about the impending economic collapse. He was a lone voice in the wilderness as he tried to expose the vast corruption within the organization.