This is EASILY boosted in ranked matches with a friend.
For this easy boosting method, both players need the "Suppertime of Destruction" DLC.
With your friend, both search on "Turret Defense (+)". This is a downloadable content game mode from the "Suppertime of Destruction" DLC. The game's online is nearly always DEAD on the DLC maps, so you should find each other with no interruptions.
I suggest using the "L'Barrage" map, which is the map with the dam, and the lightning bolt statues.
All you have to do, when in contact with your friend, just go into one turret alternately. This will get you 10XP each time you enter the turret, and also an additional 25XP at the end of the game for a win (even though you both tie).
Both you and your friend should use the helicopter, which is controlled by Woodstock (the little yellow bird character). This means you can stop in mid-air right outside of the turret door by pressing B, and whilst your friend is entering the turret rotate around so you are facing the door. When your friend leaves the turret (by pressing Y), the player entering the turret needs to press B, which stops them hovering, and sends them flying into the turret.
Also, be careful not to stop too close to the turret doors, as they will kill you if you are too close when an enemy player (your friend), enters the turret.