This can easily be boosted in Chapter 19.
Towards the end of the level you'll come across Mercenaries fighting with Descendants. After progressing past the first few mobs you'll be led up a set of stairs. Once up the stairs there will be a Mercenary on your right standing next to a crate with a Dragon Sniper on top. Once all shots are used, reload the checkpoint and you'll respawn nearby.
This can also be combined with: