Low gravity is a really disturbing phenomenon in Fortnite during which your character flies several meters at the slightest jump. Practical to avoid fall damage and to cover long distances quickly, it is however highly annoying during a construction duel and makes you an easy target for a shooter on the ground.
One of the challenges of the Blockbuster Mission is asking you to inflict 250 damages while being affected by low gravity.
The most obvious way to complete this challenge is to visit the very first rift zone that appeared in Season 10: Loot Lake. Its particular effect is to alter the gravity throughout the area.
The second area of interest is the meteor, because any player who is close enough to the star will be affected by low gravity. This is probably the easiest place to succeed in your challenge because once in the meteor galleries the low gravity won't get you in the air, so you won't be an easy target for all the other players in the area.
If you are neither in Loot Lake nor at the meteor, there is one last solution: anti-gravity crystals — brought back by Epic Games at the former location of the football stadium. Consume one to be in low gravity!
There you go. Good luck with your next challenges!