You can unlock this through natural progression of the game, however you can get this out of the way fairly quick, and can be unlocked by actions from either you and/or your teammates.
Use either incendiary ammo, the Incinerate ability, or blow up explosive canisters to ignite twenty bad guys on fire. If they have armor, then you need to cut through the armor first, which all three of the above can also do effectively.
The biotic powers of Slam, Pull, and Throw will also work if the bad guy screams, which is pretty much every time.
There is also a flamethrower (M-451 Firestorm) in the Zaeed loyalty mission which also works.