Quick & Easy MVP, I've helped many players doing it simply in Call to Arms, Using the Blackwater map.
Designate a player to be posse leader, set up the posse and invite 3 others. There will need to be another player to leave and rejoin the posse after each match. (It bypasses the cooldown timer, on the Call to Arms mode).
Soon as you are all in the posse, the leader will start the match. To start Call to Arms, Press right on d-pad, to open the Satchel, so it's: Satchel > Documents > Telegrams > Call to Arms. (If you don't have call to arms mode available. go to the post office or lockbox to pick up the telegram. (Must open it whilst on foot, not on horse).
After the Introduction cutscene has ended, everyone holds CIRCLE to skip the preparation timer, have the player who is going for MVP, kill ONE enemy then die. The other 3 die immediately after wave 1 starts, - There are red explosive boxes / barrels around the map, just run up to one and shoot.
When all players are dead, ONLY the posse leader presses the button on the "Failed" screen. Posse leader holds SQUARE on the "Abandon with Posse" option. You will then see the outro play and the person with one kill gets MVP.
After you load back into the map, the designated leave and rejoin player, quits posse then rejoins immediately. (Tap left on d-pad go to posse, find the settings and select abandon.) - Ideally to make it easier the player leaving posse should count to 10 then do it so it gives time for the leader to navigate to the telegrams page, if the page is still greyed out, have the designated player leave and rejoin again, whilst posse leader is on the map selection page. Repeat the same steps till everyone has the trophy.
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Quick Summary
1 host
MVP gets 1 kill then dies
Other 3 die immediately
Only leader presses button on failed screen
1 player quits posse and rejoins - to bypass cooldown
Repeat above steps
Hope I made it clear for you guys.