Find a partner, and join a private match together. Use the following settings:
Make sure both players are on the same team!
Mode: Uplink Control
Map: I used Silo but any level should work fine
Match time: 30 minutes (it won't actually take this long)
Friendly fire: Off
Capture limit: 10
Capture speed: Instant
One player captures, while the other player simply stands right next to him. Once the capture finishes, the player who just stood there will get points for "Major Objective Assist." Using the above settings, it will take 2 rounds for 2 players to get 10 assists. After you get 10 assists, trophy will unlock.