The infiltration is available on attack & defend on the maps Calypso Casino and Research Labs. You must be on the attacking team. The main goal of infiltration is to go inside the enemies base, grab a briefcase, and then return that briefcase to the designated drop off area.
The fastest way I’ve found to do this is to go to Calypso Casino, zip line through elevator of glass roof and then run to the vault located downstairs. Head back up the same way and make a mad dash for the helicopter. And, as usual, getting your friend to be the only member on the defending team is the easiest way to pick this achievement up.