Here's a how-to video, but also, here's a written guide:
1. Select "Campaign".
2. Select "The Adventure" campaign mode.
3. Pick one of the Final Destination matches, that will list a name of a character (like farmer).
4. Pick the opponent you are fighting as your own troop, and start the match.
5. The two identical characters will fight, and there's no guarantee you will win, so just restart if you lose.