Domination Boosting Method:
First, get a Party of 8 people, divided in 2 seperate Lobbies.
So you have 2 Lobbies consisting of 4 members each, the Lobby Leaders have to be guys that live close to each other, like 2 European Leaders, or 2 Americans etc then search for "Domination" and "Players 8".
When the game begins you will have 2 teams, Marine and Alien, and you'll find 3 Capture Points on the map. (Just leave the 3 Capture Points alone, because the Match will take 1 hour if no-one captures it, and if you accidentily capture it, tell your partner to stabilize it) As the Marine, you will get the most XP, because the Alien get's significantly less. You have to find a Partner (Preferably already discussed in the Xbox LIVE Party) and find a spot on the map where you want to meet to boost Kills.
The way you kill is this: The Marine equips his Pistol (Up on D-Pad, Infinite ammo) and shoots the Alien 3 times in the head with LT, then shoots RT twice in his body, and finishes the Alien off with a Melee hit (RB).
Then here is the key (All credit goes to DriZzT1987
!): The Alien body will sometimes start "glitching" meaning flying and moving through the area, just keep on meleeing his dead body, every scream the Alien makes is another Melee Kill! Which nets you around 10 XP.
Let the Survivor get 99 Kills first (If it's 100, the match will end), because he gets the most XP (You will have to switch sides next match, so the Survivor has to be Alien this time, and vice versa just to be fair to each other) and then it is the turn for the Alien to get kills just to pass the time. Either you hit him first with LB and then Execute him, or just a regular Execution, it doesn't matter, either way works.
The Survivor will get around 1500-2500 XP using this method, and the Alien will get around 400-700 XP.
Good luck boosting this! It will take a while unfortunatly.