The first level you can kill a demon on is OUTPOST.
While going for this achievement
You'll come up to the roof of the building where you'll find a Demon resting. Grab the sticky pipe bombs from the little hut where the tape recorder is and line yourself up with the demon. Don't get too close otherwise he will fly off!
Stick him with a well placed sticky grenade and he will die in one hit from the explosion.
After that press start and load last checkpoint and repeat again for the achievement to pop.
You can get to the roof of the building by going around the left hand side and climbing the digger and up the ladder to the roof. You can't miss the building as the compass will direct you towards it.