You'll walk past a Dropshot near the end of Act III: Chapter 4 shortly after a cutscene where you decide to head toward the Ore Transporter to go down into the mine. (I think this is the first time the weapon is available in the campaign).
Immediately after, a few enemies will spawn that can both be headshotted with a well timed drop. After running out of ammo or killing the first two enemies, restarting the checkpoint puts you right back in front of the weapon again for quick and easy re-tries. Your total won't be lost.
Tip from Dread Reaver -
The trick for this is to not aim at the enemy exactly, but to aim at the top of a box or crate that is next to the enemy. Then the dropshot should detonate at roughly head level. In this example, it's usually best to try and go for the enemy hiding at the back of the room after picking up the dropshot.