Not to take away from Jennitiger1183 but my method offers an option that makes use of the organizational aspect of live parties coupled with the horrible matchmaking to your advantage. Very similar to the domination method but it offers control over chaos.
Using a party of 4 ppl (8 will help you avoid randoms, but reduce your kill count but not significantly) enter the menus and split into 2 groups of 2. (I personally have never checked connection comparability using DM but it would prob work here).
The key here is not to play domination but instead both groups search Mixed Species DM. The reason being is often when moving on to the second game of a domination boosting session you can never get the marines and aliens to swap teams keeping everyone happy! Now once in the lobby one person from each team plays as the marine the other plays as the alien. Obviously if your playing with 8 it will be 2 marines per team 2 aliens per team.
Contrary to Jennitiger1183's calculation the Marines not the aliens will get more xp. Let me explain. The bonus will come from performing a melée kill followed up with subsequent melée kills. Perform it as such, swap to your pistol and using burst fire 3 3 rnd burst into the aliens head allowing a pause between each burst to let the alien's head come back center. After the burst quickly move to a position to the rear/side of the alien and put 2 single shots into the alien hip. Next perform a melée. If done correctly the alien should die on the first melée if not. Keep hitting it until it dies. The following is key!!! Continue to melée the corpse, if you hear the alien scream it is registered as a kill but not displayed on the in game kill count. The most I've personally achieved is an extra 5 screams!!!
To cover gameplay, the games are shorter and faster than domination. My example will use a party of 4. With one player each team as a marine one as alien the marines will come to a generally central area of the map. The aliens will run to the marines to be slaughtered. By having one player on each team the players can exceed 25 kills each without reaching the team cap of 50 kills. If your partner is fast you can easily get 30 kills my best us 34.
The end result depending on the number of "extra" kills you acquire is 400-700 xp. For 20 min of gametime. Over the course of a typical domination boosting session you WILL come out on top. Again the huge benefit is being able to alternate the team members species easier to speed up matchmaking!!!
Good luck to all if you have any recommendations or corrections please PM me! Thanks