SpecialistTHE FASTEST BY FAR STRATEGYWhile boosting BEANS BULLETS BANDAGES you mass loads of exp, when I unlocked my BEANS BULLETS BANDAGES achievement I had 300,313 experience.
Note: You obviously don't have to join a session just to get this achievement but if you want to go to Colonel, and hit the highest rank (rank 25) then you might want to join the sessions.
Here is my link for full details on BFBC boosting sessions: