This can be done early in the game. As you explore, more investigations unlock.
Go to the RESOURCE CENTER next to the bazaar in Astera. Register several investigations to Slay 20 Vespoids (mosquito looking creatures). Each attempt is considered a completion toward 50. A ranged weapon is recommended, a bowgun or bow. Now go to the nearby QUEST BOARD, choose investigations to slay 20 Vespoids, and choose to spawn at CAMP 11 in ANCIENT FOREST.
Head to area 6 and you'll see plenty of vespoids. They continue spawning from the top of the trees up on the raised plateau. It takes less than 4 minutes per run.
But, it's a good idea to work on bounties as you go. There are mushrooms, bones, plants, bugs and honey in the area you can loot along the way and during the 60 sec wait after completion. So remember to head back to HQ, instead of camp, when you see you completed a bounty, to trade it in for a new one. Plus rewarded ARMOR SPHERES for every completion.