A lot of people are having trouble with the 'psycho' skill kill. There is an easier way to get it (its still hard though).
Join a Crash Site game, with the following perks
1. Proximity Alarm
2. Stealth Enhance level 3 preferably
3. Side Pack level 2 (allows for extra explosive i.e. C4)
With these modules the idea is to run to the site before anyone gets there, plant TWO C4 and hide. With your choice of gun, somewhere safe, scout the area, wait till 3 or 4 enemies are capturing and detonate, whilst charging at another enemy to secure the 5th kill.
Mostly luck, I am currently working for this achievement and so far so good, ive gotten 4 kills and just missed out on the 5th. Good Luck