I loved RDR and would have gone for the plat for sure if there weren't any MP trophies. But since there were I didn't bother 100%ing the single player.
If we can get some CAGs to boost the matches, it's not too bad since the leveling glitch still works and most of the rest can be done solo in a friendly public free roam.
HOW THE WEST WAS WON - Reach LVL 50:
Go to McFarlane ranch in a friendly (no friendly fire) public free roam and look for NPCs laying on the ground. Shoot it in the head and the center of your screen should go blurry and a whooshing noise like the wind should start. Hunt down as many in the area as possible, but don't shoot more than 5 or 6. They will die and respawn and die over and over shoveling XP to you in bucket loads. After about an hour of idling you'll get lvl 50. After that is done quit to the main menu to make sure it saves.
Lvl 50 unlocks a lot of different character, weapon, and rideable mount options. Go into a private free roam and enter Outfitter to change your character, title, and mount (it often freezes if you try to do it from public free roam). Now that you have the best weapons the rest of the multiplayer trophies will be easier.
HAVE GUN WILL TRAVEL - Do all hideouts in a single public free roam session.
Join a friendly public free roam and do the 8 hideouts (kill a mob of NPC gang members) without exiting / disconnecting from that free roam room.
New Austin - Fort Mercer, Gaptooth Breach, Pike's Basin, Solomon's Folly, Tumbleweed, Twin Rocks
Mexico - Nosalida, Tesoro Azul
With the better weapons they are pretty easy to do solo. If you die you can just respawn and any gang members you killed will remain dead so just keep throwing yourself at them until they are all dead.
Use the wagon wheels called "Transport" on the map to fast travel to the hideouts. If you enter the map and put down a waypoint near the desired hideout, one of the options will be to fast travel directly to it on the Transport menu.
SLOW ON THE DRAW - Get 10 assists in a single hideout
With a little bit of luck you can get this without boosting and possibly during your "Have gun will Travel" circuit. Look on the map for other players doing hideouts. Once they finish one, either wait till they redo that hideout or follow them to the next hideout. Use the Evan's Repeater as it can fire quite rapidly and has a large "clip" of 22 shots. One shot to the chest is also not enough to kill with it.
So as soon as the hideout attack starts with the other player(s), point the cursor in the general location of an enemy, then hold L2 and it will auto-lock onto their chest. Fire one round to damage them, then release L2, move the stick to find another target and then hold L2 and go for another chest shot to damage them. Do that to as many enemies as possible as fast as possible.
Tumbleweed, Soloman's Folly, and Tesoro Azul are good candidates as the enemies are all bunched together and the win condition is to kill all enemies.
HIT THE TRAIL - Ride from Blackwater to Escalera between sunrise and sunset
Join a friendly public free roam and fast travel to Blackwater in the NE corner of the map. Press start to check the time, if it is nowhere near 6 AM join an different public free roam. Once you find one that is fairly close to 6 AM without going over, wait in the Blackwater on your mount until after 6 AM then ride for Escalera. If you put a waypoint direct on Escalera then it sends you a convoluted route through New Austin. Set a waypoint around Casa Madrugada then once you cross into Mexico change your waypoint to Escalera and then ride hard.
Pressing X, X, hold X for a few seconds seems to keep the horse at top speed without depleting much stamina. With the fastest horse mount (pick the one to the left of the bull Bonzo) you should get there in plenty of time.
MOST WANTED - Once you reach a bounty of $1000 for killing NPCs, stay alive for 10 minutes
Enter a friendly public free roam and stock up on ammo and find a decently defensible place with NPCs (I picked Armadillo). Kill an NPC and lawmen will start to arrive to kill you, keep killing them until you reach a $1000 bounty and become "public enemy." Call your mount and then ride hard out of town until you lose your wanted status (you will still have the bounty but lawmen won't be chasing you). After 10 minutes the trophy will pop.
I set up shop on the front balcony of the Armadillo Saloon. From there I had cover behind the railing and when all the enemies were dead below I could watch the mini-map and kill lawmen with the sniper rifle as they approached from the edge of town.
READ DEAD ROCKSTAR - Kill a rockstar employee or anyone who has the trophy for killing a Rockstar employee.
Join a normal public free roam. Press select and then R1 to see the list of players. Look for literally anyone who has a level greater than 10 or so, track them down on the map and kill them and you will probably get the trophy.
One's you will probably need to boost:
POSSE UP - 7 other people accept your invitation to join your posse for a total of 8 players. Invite players from the players list which you can reach by pressing select then R1. Only the leader gets the trophy.
THE QUICK AND EVERYONE ELSE... - 3 consecutive wins in FFA in public free roam
Find an underpopulated public free roam (best done at non-peak hours) and invite one or more boosters into the game by pressing select, R1, then triangle to switch to friends list. Posse up and then go to a big town like Chuparosa away from players and join the FFA match, select Goldrush with the d-pad, and vote to immediately start.
Have player 1 win the duel, then have player 1 and everyone else start collecting bags, player 1 caps bags while the other players stand next to cap points and let player 1 kill them. This drops their bags so P1 can pick them up and cap them. When 10 bags are capped P1 wins. Before capping the last one, make sure no other players have entered the lobby. If they have all players should kick them. Do this 2 more times.
GO TEAM - 4 Consecutive wins in public team Match
Same as above, except go to Gang Match (it can be done with 2 players) and do it 4 times instead of 3 times. Select the game mode Hold Your Own (capture the flag). Teams are likely shuffled each match so probably only one person at a time is going to be able to go for the trophy.