Here is the easiest solution:
On the level Outpost there is a demon on the roof right beside the radio tower where you can broadcast the Commander's message.
To get there without being noticed by the demon simply jump on the excavator on the left side of the building and then take the ladder to your left.
You will find 2 sticky grenades under the lantern in the radio tower. Pick them up and sneak behind the demon. You should be able to throw at least one sticky grenade at him - this should take him out on lower difficulties.
Then simply reload last checkpoint and repeat the process another time.
Good luck!