Can be done in any campaign or custom gametype. This can also be the same enemy that has respawned over and over. I found this easiest to do in CTF. Set up a CTF match with both teams set to 1 player, set the time limit set to 0, frag limit (which means capture limit) set to any number, Difficulty set at the lowest level on the difficulty slider. In CTF matches, the enemy planes will generally fly straight forward toward your base, making it very easy for you to shoot them down repeadedly. Kill the enemy 10 times quickly enough and you got it.