***If you vote negative, please comment as to why, it's my first guide, so constructive criticism is appreciated. ***
I no longer have this game, so I can't help you boost, sorry.
Download Complete Boosting guide
You must get 650000XP to reach level 50.
For a breakdown of the math of this method, check the comments below.
Credit goes to: The mystery people who told ToughICNJ24 who in turn told me about it. Also Floppy Chong, ultimatedra, xKarxFreddy, and several others who helped me prove it worked. Also Zacry072 for his poison tip!
The simplest method to boost XP in AC: Brotherhood is to have 6 people enter an Alliance (unlocked at level 5) game in Player mode.
At the start of the game EVERYONE runs to a central location, then you divide in to two groups of three, with one person from each team per group. Lock on to the person chasing you, and stun. Then the person you're chasing stuns you, and is in turn stunned by the person you stunned. It works out to two triangles of people just punching the crap out of each other nonstop for an entire round. At the start of round two run back to the exact same spot, and now comes the hard part....knock out the person who knocked you out in round one. Yep, that's honestly the hardest part...but really, the entire method consists of lock on, push B, get up, push B, get up, push B, repeat ad nausea.
A few tips for making this go smoothly, at level 27 Resistance unlocks which reduces your knocked out time from 5 to 3 seconds, making this method even quicker. DO NOT use disguise, morph, firecrackers or anything else that can break lock because it slows everyone down. Mute is okay, I personally thought smoke bombs screwed up the rotation, because that second you are coughing is a second that affects everyone. Keep your fingers off the X button, as an inadvertent kill will cost your fellow booster about 900 points given re-spawn and return trip times.
NEW TIP: Zacry072 brought to my attention that @ lvl 29, all players could equip Poison and the x2 bonus for 5 contract losses. Near the very end of each round (approx. :15 seconds left), break lock on your pursuer and quickly poison your target for a nice overall increase in score. Zacry072 noted that he was averaging 17-18K per game, and players not doing so were floating around the 15-16K mark. So, lvl 29's and above, MAKE SURE YOUR FELLOW BOOSTERS KNOW THE PLAN and then poison them with 15 seconds left (everybody who has poison should be able to get one in, even if they are themselves already poisoned, before it takes effect) for a great score boost. Thanks Zacry072!
Good luck, hope this helps.