A good Primary of course! Miniguns are used for anti aircraft fire irl for a reason, for instance. (Although I find that my Royal Fighter works very well too!)
Reflector and similar high damage weapons also work.
Weapons with travel time are the least suitable, unless it is fairly close range, and you have enough practice!
Try switching to a weapon with a slow down attribute, that should make them fly much slower.
Or, a fast firing gun to spray at jetpack users.
You can also rocket jump and quick-switch to a high damaging weapon with fast bullet travel time, like the reflector, or royal revolver
big hitbox weapons+fast flying (fidgetspinner/trident/syne/...)
What do you think?