Knock down 6 or more trees with 1 explosive. Alone in Single Player or together in Multiplayer.
You just have to get lucky with the tree placement, I guess. You can make IEDs (improvised explosive devices) by mixing tape, batteries, circuit boards, watches, coins, and booze. A lot of this can be looted from the crashed airplane you spawn in and the various luggage containers.
I don't know if the world is the same for every player. Even if it is, I, unfortunately, don't know where to obtain the circuit boards or tape.