The multiplayer is dead, so the best way to get this is by boosting.
You need 4 people to start a match in this game.
The best boost method is called the suicide method, and can be done on all maps except for Constantine or Manhattan. Once you gain a few levels, make sure to change your loadout to all Kehei armor on all slots, remove all your weapons first, and then for your right arm under "weapons", choose the Winee rifle.
Simply get your group of at least 4 people, split off into pairs when the game picks the teams (one from each team paired together), and have 1 player designated as the Suicider, and the other as the Killed.
The Killed person first starts by shooting the arms and legs off of the Suicider. After that's done, the Suicider shoots off the arms and legs off the Killed, and then finishes by shooting the torso of the Killed.
Immediately after killing the Killed, the Suicider then falls off the edge and kills themself.
By doing this, the Killed also gets equal credit and XP for the suicide, thus everyone gets equal XP.
Note: Don't pay attention to the bonus XP notices on the screen. They're not accurate, and you'll receive bonuses even if you don't see the notification for them pop up on the screen. The best way to tell if you're getting XP is to look at the scoreboard by pressing the select button.
Most of the multiplayer trophies can be done with this method, and you'll unlock:
by the time you reach around Rank 55, so no need to waste time boosting that one separately.
When you're ready to end the match (since it won't end and will continue going into Overtime if no one makes a cap), simply designate a winning team and have them score a cap. Once the remaining time runs out afterwards, the match will end and designate them the winner. This can be used to boost
In total, this game should take about 12 - 15 hours of optimal boosting, so long as everyone works together.