Theres another very easy level to get this achievement on, if your a bit terrible at finding the secret areas like i am. if you wait until you reach the level Argent Facility (destroyed) the three challenges are quite simple.
1. Skeet shoot: Kill a Revenant whilst it is in the air. you encounter a few of these early on in an open air part of the map and they like to jet-pack around i recommend using a rocket launcher hit them a few time then wait till they fly hit them once if they die fantastic if they dont they will land run around a bit then take off again, once again shoot them once until the challenge is done.
2.Close encounter: kill 5 demons with a chainsaw. i Highly recommend using the chainsaw on imps, possessed soldiers or or the possessed as these only use 1 unit of chainsaw fuel. rinse and repeat.
3. Knowledge is power: find 3 data logs. there are a few of these in the office areas during the first part of the map make sure you thoroughly search each office and corridor and you will find them. i did not need to find a secret area to find 3 of them.