Easily done with 2 people after the latest patch. Have a friend in chat and on the main menu arcade screen hit Square for private. Enter simple password and tell your chat friend to enter it on their end.
For map select Neon Gladiators- 5min round.
Choose who will win, make 11 kills.
Let your friend get 10, kills add towards the 100 kill trophy.
Repeat process 10 more times, trophy pops on victory screen. It may have to be 11 wins depending on the FC servers.