This can be boosted with 3 controllers, or with 2 friends (or 1 friend with a 2nd controller). Remember that it has to be a public match. Start up a public match and ready up before randoms have joined. Now with the account going for the achievement, pick up a bow and some arrows from chests. It takes roughly 4 arrows to kill an enemy. You can damage them with a sword and do the final blow with bow and arrow. If you run out of arrows keep searching the chests as every now and then new items are added.
You can of course also do this in a public match, though that'll be much harder.