This achievement asks you to reach Rank 5, which is the max rank obtained at level 20. To get this rank, it is a cumulative 300,000xp.
12,000-16,000+ XP every 10 minutes.
The information below is also clearly (and visually) explained within the video located at the bottom of the guide.
If you select "Invite Only", you can gain an extra 20% (2000+) at the end of the match.
Make sure you are a heavy class, with the Chinzor MG as a primary weapon. Add attachments to suit your personal needs. Buy the Caltrop grenade for the Operative class, if possible, and set Operative as your default class.
Start the last Security Mission (Day 8: Fallout), on solo easy. As an operative, make sure you select "Defend Vent System" on your objective wheel, and run to the room where the vents are located. Now you will wait until enemies come to attack these points, and whenever you kill them, disguise yourself for a 125xp bonus. You will also be getting "defending" points for being near the objective. Keep running between the two rooms, and disguising yourself whenever you get the chance. Try to stay away from dying. Also, make sure to throw a caltrop grenade into the doorway of one of the vent rooms, to get points when enemies walk into the room. It will also warn you that they are coming. You should be able to about 1,000 xp every minute, and get a bonus for being successful (15%). At the end, just "retry" the mission if you want to continue boosting.
Video Solution Guide: