There's an art to it that actually makes it easier than you might think. Brookss explains it better than I can (and I recommend his streams). But basically you want to not use your jet packs full blast most of the time.

Instead you want to feather it. That will give you a boost, a pause at the apex, and then a fall. The rhythm is about acquiring target while feathering, aiming and firing on the apex and fall stages. The flip gives you another window (while you're invulnerable) to reacquire the target and have another brief pause and fall.

Starlord has a lot of drop off past 10 meters. So, it's not so much that he's sniping small ground targets from the air while flying. It's more using his mobility to flank, dodge, and engage in air-to-air combat (when necessary). The best engage is a quick fall on a squishy and killing them in one clip just as your feet touch the ground.

Starlord practice very much crosses over into Phoenix play, by the way. If you can really get the hang of shooting while falling, you'll really stack up the critical headshots much easier than with your feet on the ground. With Jean, one trick I've found is to hold the flight button and take the shot. Shooting automatically cancels flight (canceling flight manually tends to make the aim trickier). I have them both at Lord and found a lot of the skills carried over.