To unlock this trophy, you have to join and complete only one match in an online tournament match.
First, you have to decide whether or not, you can do this trophy, what I mean is that you will have to have either one of these:
-Two PlayStations with two copies of Rocket League and two controllers for each of them
-You have two controllers and your friend has two controllers as well
-You can have three friends join you
If you have any of them available, you can do this trophy.
To start off, we need to press "Play" in the lobby and go to "Tournaments". Press to create a private tournament and use these settings:
Team Size: 2v2
Bracket Size: 8
Standard Match: Best Of 1
Joinable By: Name/Password
Once you're done, you can share the name and password so your friends or yourself can join, then just score one goal and earn this trophy.