After each mission (on Veteran difficulty) you are scored between one and three gold medals. For this achievement, you need all three gold medals on all six of the Assault Missions. This can be done in single-player, local co-op, or online co-op. You can check your progress in the Achievements menu on the Main Menu, or on the mission select map.
In Bully Fight and Zeppeligeddon missions, you can get bonuses for finishing quickly and not dying. In some Assault Missions, you can get bonuses for killing enemy officers. In Bombs Away missions, you can get bonuses for destroying all the enemy ground targets, as well as bonuses for killing large numbers of enemy planes (they endlessly re-spawn).
Most of the Assault Missions can be easier with a partner. If you are having trouble getting all three gold medals on any particular mission, try looking for a partner online.