You can do this in a multitude of ways but the easiest way has to be obtaining a shotgun in the early levels and then going to the Black Market to mop up the kills.
You can steal a shotgun from a shop easily by picking it up and shooting the shopkeeper before he can react. On the next level and subsequent levels, there will be a shopkeeper guarding the exit, but if you approach the exit carefully you can often take him out by tossing bombs (sticky bombs are the best for it) or waiting for the right moment to jump in and shoot. As long as you hit him once, you're likely to win as he drops his shotgun when you do. Watch out as the shopkeeper is coded to fire the instant he has you in sight, and there is no window for you to react. You need to create a situation where you're absolutely sure that you can shoot first.
You get to the Black Market by finding a hidden exit in the Jungle. You can find the exit by obtaining the Udjat Eye from the Mines levels, by using the key on the locked chest. I believe the Black Market has 8 shopkeepers so if you got the shotgun on levels 1 or 2, you're almost guaranteed the achievement if you can kill most of them. A good strategy for the Market is to pop a rope right where you spawn and then climb it and wait for shopkeepers to kill each other off, or have them gather up at the bottom of the level to the left or where the Ankh is - both spots seems to "trap" the shopkeepers pretty well. The Ankh shop is the best spot since you can just stand up to the right and toss bombs down there. If you have stickies, you can jump, press down and toss and if you aim right, the sticky will bounce off the wooden ladder and land further down, killing most or all shopkeepers down there.
It seems that, just like the fact that anything that hurts the shopkeeper makes them mad at you even if you didn't actually hurt them yourself, you also get credited for the kill if they kill themselves or each other. I had my achievement pop when I wasn't fighting any and was just heading to the exit of a Jungle level.