In the OBD, the word "solo" usually refers to a character single-handedly defeating a team of enemy characters, or an entire fictional universe. For example, if a match is made between a team consisting of characters A,B,C, and D, and another team consisting of characters E,F,G, and H, someone might say that "Character B solos", meaning that Character B can defeat the other team all by him/herself without any help from characters A,C, or D.
For example: "Enel solos Narutoverse", "Galactus solos DBZverse", or "Hulk solos OPverse".
Note: Not to be confused with the movie of the same name.