You may be able to complete this on your initial playthrough, but you would probably have to hit every single saucer that appeared. The fastest way to farm these kills post-game would be to quit your second playthrough, not load your save, and start on Titan. This first stage is relatively simple, and spawns 3 saucers every time, but DO NOT FINISH THE LEVEL! Once you get the third saucer, simply save and quit, then reload your game.
This is also a good level to farm alien captures as you will also get 2-3 each time.