This is a fairly easy achievement. Half way to your first waypoint you will be told that Zeros are on your tail. Press to switch to your gunner and mow them down. If you don't get the two (2) kills at this point you will have one more chance later in the mission.
After you torpedo the Japanese ship and are making your way to the waypoint in order to escape Zeros will come down on you again. Again press to be the gunner, and this time around there will be significantly more Zeros, and they will be much closer to your plane.