Very easy if you do this.
Area - Do Not Set
Opponent Strength - Beginner
Battle Record Status - New Opponent first
More of a note than a full guide. It has to be 50 unique warriors. You can't just defeat the same character 50 times. Therefore if you fight anyone multiple times you can't use your win/loss counter to track your progress anymore. hence why I recommend hitting the new opponent first. If you are unsure the people you have though show up in white text rather than green and say your previous battles to the right of the screen. (Well you can but just remember who you've fought multiple times)