A One Character Victory is when you win using only one character, the one you started with. Do this 50 times with your last 20 unique teams. View more details on unique teams in Step#3 of the Roadmap.
This will be quite a challenge, i cant give you a specific tactic besides train. You cannot switch characters however you can use assist. I recommend using your best character and the best assists that fit your play style. To keep track of your One character Victories you can check under gamercards HERE.