For this one just run into a group of guards. Kill all but 2 of them, then run away and make sure they are chasing you. As you are running more guards will start chasing you, rinse and repeat, till the achievement pops.
Note: Be careful about kicking to much guard butt, as if you are killing them to quick the last one might run away and break your "chain" thats why you want to keep at least 2 or 3 alive at all times.
Also a great suggestion by Thooper: Head to the Southwest camp on the main Kingdom Map, you will know if its the one as it has a huge pit. Kill the archers then just fight the soldiers to your hearts content, as they continually respawn inside the camp.