Start up the 2nd Mission "Halo" from Halo: CE on Easy
Right after you start up the Level and crash onto Halo's surface, take a right and pass the big bridge
. Start heading up into the mountains and you'll notice two Banshees flying towards your direction.
Pull out your Pistol and start shooting one of the two - although you'll know you are hitting them as soon as your Crosshair (the round one :P) turns red
, you can start shooting before that and your bullets will still hit'em.
If you feel lucky, try to shoot down the second banshee as well
- it will start flying into the opposite direction of your path and vanish behind the Cliffs
. This should be pretty simple on the Easy difficulty setting though.
Rescue the first set of marines until you get your warthog deployed and then rush on through the level till you exit the cave complex
.Head to your right
where you will enter a valley-like opening with the first set of Survivors hiding in the rocky hills
. Instead of defending them you can "kill'em all" (no pun intended) like I did in the Video below, just to speed up things.
After that, head back out
and you will see two Banshees circling the sky and starting to attack you. Do the same as before and - wether you shot down only one or both in the beginning - the achievement shall pop up after the first or second kill
Hope this Video can help: