As the game tutorial may have taught us, to bring a flying monster down you have to carefully aim the crossbow at it and shoot. This is kinda tough when the monsters are flying fast and you have deal with to re-adjusting the camera view everytime they swirl around.
A much simpler way to deal with this problem and drop flying monsters like Sirens, Harpies and Ekidnas is simply to tap the Use Quick Access Item button! Meaning, you don’t actually have to aim down the crossbow.
As long as the target is active on the monster (meaning the health bar shown) you can simply tap the Use Quick Access Item button immediately and it automatically locks on that monster and bring it down. This is very useful when there are multiple enemies surrounding you and you don’t want to slow down your movement by aiming down the crossbow.
This also works with underwater monsters like drowners! If you see one is approaching you and is on active target, just tap and fire the crossbow! You don’t have to aim.
This method also works with bombs. However, bombs don’t home on the target. So best to throw them while the enemy is standing still.