Either find a friend or a stranger that owns the game and start up a lobby.
You should go with the following settings:
Rounds - 1
Time - 30 seconds
Health - Smallest
Then just chose random characters or two characters that hit strong and take down low health quick, like Gen, then chose Dojo. From there, just have one character beat the other up for one round, then switch it up in the next fight. Keep doing this until the trophy pops for you, then let your friend or the stranger work on getting the trophy until it pops for them as well.
PS: Here is an added tip from Floorkiller74.
“If you did this on DOA5U on ps3, and transfer your save, it pops after your first game.“