Start a offline casual match with easy
Bots on any map.
Buy a Desert Eagle / M4A4 / AK47 - shoot all the bullets except one and then take your time. The bots are unbelievable dumb and will stand the most time around or crouch in some corner. Pick one out, aim and put your last bullet in his head.
If you still got problems with this, start Shorttrain on demolition as CT, buy the Desert Eagle, fire 6 bullets and wait till a Terrorist is planting the bomb - headshot - achievement. Shouldnt take many tries.
Gives you also one award towards your 100 for