Fortnite Chapter 3 Season 4 requires you to defeat a Timber Pine and dislodge a Runaway Boulder in a single match in exchange for a tidy 20K XP reward. The best option will depend on your Battle Bus route, so stick for the first.
In Fortnite Chapter 3, Season 4, where will you find Timber Pines?
Timber Pines can be seen anywhere around Logjam Junction or the Rave Cave, or west of Shifty Shafts. They are much larger than regular pine trees and have less snow on their needles, so you can knock them down with vehicles and other weapons.
Runaway Boulders are a game developed by Fortnite Chapter 3 Season 4.
The Runaway Boulders in Chapter 3 Season 4 are centered on Peril Pass, a canyon northwest of the Reality Tree. The Runaway Boulder will likely land on one of the geysers down there, but it's fun to watch because of it. This has nothing to do with the Zero Week quest, although there'll be a quest involving this exact phenomenon in the future.