In order to get this achievement you need to use either the front guns as the pilot, or the port or starboard gun placements. Aiming with the pilot gun is a pain because you are also trying to fly and aim at the same time so I recommend the port or starboard gun placements.
If you are having difficulty hitting anyone with the port or starboard guns, leveling up your gunnery Intrinsics to rank 2 helps a lot, it gives you a 360 view of everything around you and makes it far easier to kill stuff if you have a sporadic pilot.
You get Intrinsic points as you play Railjack missions, it requires 3 points to get rank 2.
To Level up Intrinsic you must be in your orbiter pause the game then go to profile then Intrinsic.
Kills while piloting crewships also count.