The most effective way of quickly Leveling in Red Dead Redemption Multiplayer Free Roam is by quickly replaying the Gang Hideout missions.
I recommend leveling in a Private Free Roam session by yourself. You can level in a public session or with friends but the more people that are in the game, the more likely they are to kill you or mess up your kill chain bonus.
I recommend using the Bolt Action Rifle to clear the Gang Hideouts. You obtain the Bolt Action Rifle in Tumbleweed so that should be the first Gang Hideout you transport to and clear out. After you have obtained the Bolt Action Rifle it is time to go to Twin Rocks. (For PS3 I have heard Solomon's Folly is faster leveling than Twin Rocks but I have an Xbox so I wouldn't know.)
Twin Rocks is the easiest gang hideout to level on because it is quick (can be completed solo in under 60 seconds) & efficient (you can easily chain 22 kills together)
A gang hideout that you can get 1000-2000 EXP in under 60 Seconds? CHECK!
Now, how to get the most EXP from a Twin Rocks run. Start off by setting your targeting mode to Casual or Normal. Normal will get you an extra 100 EXP as an aim bonus but if you are not skilled at re-targeting players I recommend going with casual. Expert is not recommended due to the amount of dead eye you will use to get kills but if you are really good it will give you around a 350 EXP targeting bonus.
After you have your targeting mode set, go to the mountain directly above Twin Rocks to start your run. The first bandit I recommend shooting is the one closest to you on the tall Rock. This "sniper" has a great vantage point to hit you so I always kill him first.
After you have killed him, you want to "auto-lock" every bandit below the mountain as quickly as possible. I slide down the mountain while shooting them. The bolt action rifle should be a 1 shot kill (2 shots maximum). The bolt action rifle has 5 shots so after you shoot off your 5th shot your character is going to start to reload. Rather than allowing your character to reload the gun normally, activate Dead Eye and paint one "x" on an enemy and then fire. Using Dead Eye immediately reloads your weapon and is a trick that very few players know about to consistently shoot your targets.
As soon as you kill the initial spawns, reinforcements will come out of the second building and should funnel directly to you in a straight line. This should be the easiest set of kills that you ever get. After you have cleared the "main circle" of enemies, 4 reinforcements will come in from around the town. Immediately head towards the nearest bandit to keep your chain kill bonus alive. Lock on to the remaining 3 bandits (using Dead Eye if they are out of casual lock range) and finish the run.
A typical Twin Rocks run should take between 50-120 seconds & should net you between 700-2000 EXP. AFTER YOU FINISH THE RUN, it is very important to press D-Pad LEFT to replay the hideout. Replaying the mission will save the game & your experience earned, and then allow you to restart the hideout. This is the best way to do run after run after run very quickly and level up extremely fast.
I just got the game Saturday and I am already level 30 (and I only started doing runs yesterday)
I have an HD Capture Card that I have recorded a lot of these runs with. I will be posting them soon to my youtube account so you can see how to do it in real time. I will also post one run with an audio commentary for those of you that can't understand my text.
My Channel is http://www.youtube.com/user/xiTradeMarkix and it is full of MW2 videos right now but I will soon create a RDR playlist with videos for leveling help as well as tips for completing challenges.
Sincerely hoping a Jaguar anal pounces you, XI TradeMark IX