You must reach and finish Stage 4, Wave 4 and made it alive to the chopper. This is far the most hard to get (at least for me). Getting it with 3 other friends that know how to play the game could be easier, but still is hard to survive. After stage 1, enemy vehicles start to show up. First map is easier since its smaller and the enemy only comes from one direction. Also, if a player dies, it respawns after 15-20 seconds or so, but if all team dies at the same time its mission over. For more info check this; also some tips are in the manual.