You can easily get this during the campaign. You'll first get the Scorcher right before turning on the power in the facility in Act 3. From then on, you'll face only Wretches for a long period of time. Simply use the Scorcher on every Wretch you see, they'll go down incredibly quickly on Normal difficulty. You should find plenty of ammo as long as you explore the side rooms a bit, and remember that a perfect active reload on the Scorcher gives you a longer flame, so you can start hitting the Wretches from farther away.
If need be, after you shut down the AI, you'll fight a bunch of Sires. They go down very quickly as well and rush you like Wretches.