For the Leaderless, They Fall Bounty in Destiny 2, you will need to kill twenty Fallen Captains. You can kill them by whatever means you like, so feel free to break out heavy weapons for this one, or a good Sniper Rifle.
The best place to go for this Bounty is Trostland on the EDZ. It has three Lost Sectors close to the Church that all contain a Fallen Captain as a boss. Just make your way to the Atrium, Widow’s Walk, or Terminus East Lost Sector to find a comfortable Fallen Captain to kill.
There is an even quicker way to do it, however, although it can get a little dull and repetitive. From the church, make your way to the right, up the side street, and in the broken building that leads to Maevic Square.
At the center of the square, you will find a Fallen Captain, that is very easy to kill as he is not even a Major. You can quickly take him out, then run back the way you came until the location name Trostland pops up on the screen.
Turn around and run back to Maevic Square, and the Fallen Captain will have respawned, and you can quickly take him out again. The procedure mentioned above is the quickest way to do it, but it can get a little dull. If you want to break things up, head for one of the Lost Sectors and kill a Fallen Captain there, before heading back to Maevic Square.
You will also need to get forty-five Sniper Rifle kills for this Bounty, so you may as well knock them out while you are running around.