In my opinion, this is one of the toughest medals to get, but certainly one of the most rewarding. As many other guides would say, preferably join 'Pistol Only' matches and play from there on. But the problem in these matches is that many players who already have M4's play there too, which gives them an unfair advantage. But there are four ways to solve this problem:

1) Try and find matches that disallow M4's

or

2) Pwn their asses! Mainly the method I'd use is basically use many evasive maneuvers that will knock off their aim and give you more time to get in those 3-5 shots. I recommend strafing from left to right repeatedly while also crouching midway and jumping as well. This will help as the M4 has a slow fire rate and a smaller magazine, which you can use to your advantage.
You could also hide and wait for an enemy to run by and then either melee or shoot him (although hiding is considered to be 'noobish' ).

3) Join any match type, and go to an enemy spawn point (preferably in a small area), crouch right next to the orange gas and constantly press the melee button, killing most of the people that spawn there.

4) If you want the medal desperately and cannot obtain it through normal circumstances, then you could create a private match with a friend and take turns getting the medal. But for this method, I'd recommend trying either to get headshots each time (that will get you another medal as well) or melee your friend with the pistol (which counts as both a pistol as well as a melee kill, again getting you two medals). However, if people find out about what you did, you will be frowned upon by the rest of the killzone community.

*note; if you can, use the assault class with the health pack ability as you start off with 50% more health which at times will save you!

(*just so you know, I used the 1st and 2nd methods ONLY, I haven't cheated and I don't think I will )