I have tried to boost & gain experience by using several different methods in this game. I will list what I have found and believe to be the most effective, as well as other attempted and suggested methods that I have found to be less effective.
I believe that the most effective way of gaining experience to reach level 55 (526,000 XP) is by playing private - wolfpack unleashed - solo. By playing every day, you gain a double xp bonus for the first 20 minutes of game time. After one or two attempts, I can now consistently complete wolfpack games in around 15 minutes. The 20 minute double xp bonus carries through a second game as long as you start the second game with the bonus active.
For the last few days, I have consistently received just over 40,000 points per game (would be only 20,000 without the bonus), which turned into just over 80,000 points per two games. This took me around 30 minutes of total play time. The way that this is done is by completing as many secondary objectives (ie. perform an acrobatic kill, perform a locked kill, etc.) as possible while playing wolfpack, as well as getting as many full focus (let the assassinate button 'circle' fill up), hidden (blend in with the target's crowd), and perfect approach kills (allow the meter above the target's portrait on the lower right of the screen to fill up) as you can. You should also complete the one-time challenges for completing the wolfpack unleashed game on each map, as well as the one-time challenges for playing as specific characters.
In the later rounds of a wolfpack unleashed game, the target almost always has a look alike in the crowd with him. Because of this, I found the most useful abilities to use while playing wolfpack are the ones that allow you to pick your target out of a crowd (firecrackers, money bomb, etc.). I would also recommend the resistance perk, which helps if you ever become stunned. Knives are my preferred ranged weapon, because there is not a 'lock-on' time like there is with the pistol. You can still complete the 'Kill a target with a ranged weapon' secondary objective if you slow the target with a knife. You should also use either the 250xp or 550xp kill streak bonus (250xp bonus unlocks much earlier than the 550xp).
I will list what I found to be less effective ways of ranking up and earning xp below. These are still faster than playing multiplayer legitimately, but I found wolfpack to be the fastest.
Stun boosting via manhunt games or domination games. This is done by forming two clans with 2-4 players each (all eight will fit in an xbox live party). The clans can then search for each other in adversarial - clan - manhunt / domination and take turns stunning each other. In manhunt, the hunted team stun the hunter team. Halfway through the match, the hunters and hunted switch sides. In domination, each team captures the initial point at their spawn. From there, team one will run to team two's side of the map and stun team two for the remainder of the 10-minute game. I found that these methods gave each person around 8,000 points in every 10-minute manhunt game, and around 17,000 points in every other 10-minute domination game (without the double xp boost).
I found one listed suggestion to gain xp faster was for the team being stunned to revive each other. The people that I boosted with, and myself, had issues locking onto targets with everyone standing in a close proximity, and the game would not allow us to stun each other. Because of this, we were forced to pair off away from each other so that the stun boosting could continue.
Artifact Assault was another method that we found to boost xp. This was also done by forming two clans, but with only 2 players each (all four will fit in an xbox live party). The clans can then search for each other in adversarial - clan - artifact assault. This game mode is similar to a capture the flag game, though your flag/artifact does not need to be at the base in order to capture the enemy team's flag/artifact. Due to this, each team would run to the other person's artifact, grab it, and run it back home for the capture. The first person on each team to arrive at the artifact would grab it and immediately run it back. The second person on each team would wait at the enemy team's artifact, because it respawns as soon as it is captured. As soon as the artifact respawns, the second player would immediately grab the artifact and run it back to the base, while the first player was on his way back to grab the artifact again. I found that by doing this, each team would get around 8,000-9,000 points per 10-minute artifact assault game (without the double xp boost).
Please let me know if you have found issues with any of this and I will try to clarify. Happy boosting!