There are several Demons in the game, be prepared if you decide to attack on in the open. You will be able to kill two Demons will little to no effort on the chapter "Outpost", once you climb to the roof of building you can hide in various doorways or the wooden shack with the broadcasting equipment.
Once you find the shack on top of the building you have two options, standing in this shed will protect you from the Demons attacking. You can stand their with a weapon of choice and keep shooting until both Demons are dead.
Another method is to use the sticky grenades which will find the shack. These grenades will kill the Demon with one grenade, though you have to time your throw and aim carefully. If you're not so good with throwing grenades just keep shooting the demons until they die.