This challenge is part of the Midas Mission Challenges, the ultimate agent Fortnite Chapter 2 Season 2 focuses on. You'll have to deal damage to a helicopter with at least one pilot or passenger. The challenge doesn't explicitly refer to a helicopter but Choppas — the term used to describe these flying machines, which were introduced during Chapter 2 Season 2.
Helicopters can be found in Solo, Duo or Squad modes.
If you ever have trouble finding one during your game, the easiest way to complete this challenge will be to camp close to the helicopter tracks on the map, then wait for a pilot to borrow the craft. Then open fire!
Map
Only important detail: Choppas are fast machines, and not all weapons are good to use here. We warmly recommend to shoot at helicopters with the Assault Rifle.