Before playing in modes such as Arcade, Versus or Training, you'll be given the option to have throwdowns "accept" or "decline".
For this trophy you will need to have accept on.
*You cannot be playing with a friend locally. As throwdowns only occur when you're playing solo*
While playing Arcade, Versus or Training, there will be moments when you get a fight request in the bottom right corner of the screen.
This is indicated by a Red or Blue envelope.
Red means you're being challenged & Blue is you issuing a challenge.
When you see that envelope press the "select" button.
If the challenge issued is accepted, you'll be thrown into an online player match.
Upon completing the match you'll earn the trophy.