after saving Falco you have to fly under all of the arch things. After this Falco will lead you under a waterfall. There are more kills this way and you can choose which level you want to go to when you get back to the map of the Lylat System by simply hitting A once on sector Y then a screen will come up with a switch course button.
I recommend doing the levels in this order if you are going for the greatest amount of kills
2.Meteo (Asteroid Field, take the shortcut into hyperspace)
4.Sector X (Dont take the path into hyperspace instead attack from the outside with Peppy)
5.Macbeth (Train Station, be sure to hit the train into the station for 50 extra hits)
7.Venom (With Starwolf)