After struggling to find the Dragonfire shotgun at recommended locations or anywhere in game I tried a different way, which requires a bit of luck with cards received in packs.
As always start an expedition in the cold darkness game, go for Adventurer for difficulty to make it easier for yourself. On the next screen make sure to have the ‘Give a Man Fire’ challenge equipped.
On the next screen press add a card and go down to Player strengthening, scroll across (they’re alphabetically ordered) looking for ‘Cat and Mouse’ and select. This gives all shotguns dragonfire rounds, but enemies evade more, not a problem on lowest difficulty. The luck bit comes for getting these cards in packs, I had 2 from playing campaign and 1 dlc. Then scroll up to the shotgun selection cards and pick one you fancy, I went full auto. Then start the expedition.
Once started just kill enemies using
to fire whilst aiming, you start with some dragonfire ammo. Make sure to loot enemies and you should easily have enough ammo/ingredients to make more dragonfire ammo, by pressing
when not aiming.
It only takes a few minutes to kill enough enemies and then your Achievement should pop.
Good luck with the Achievement.