Since I haven't seen this posted here I will post it to help others as it seems the fastest way to earn this achievement. Note I haven't earned the achievement yet but using this method nets 74-78 thousand exp per match so I should be done soon enough
.Also I didn't invent this solution I heard about it while boosting other achievements.
First off this solution will require two people so find yourself a boosting partner. Then set up a match with the following parameters.
Type- Free for All
Map - Underground
Time - 20 Minutes
Kills - 100
Character - Any of the survivors as the Solarii do not get a grenade launcher.
During the match you will both want to run onto the ship in the middle and stand in front of the sandbags opposite each other and ready your grenade launcher then fire it at each other this should kill you both within seconds of each other netting you 600 exp for the double kill plus 300 for revenge and another 150 + 75 for the explosive and kill doing this for 20 minutes will net you 70k-78k maybe more if you manage more kills. Doing this I average between 55-65 kills a match. When the match ends exit to the lobby and recreate the room rinse and repeat for some quick and easy exp.
Again I take no credit for this method I just wanted to post it to help others out.