It is recommended you combine this achievement with "One Guy Army". Before anything else, make sure you have no quest active or the achievement won't pop and make sure you are prepared. Get yourself some armor, a shield, a sword, and lots of bolts.
To make your wanted rating go higher, you need to kill the guard because killing civilians will take forever to get to the maximum wanted rating. For this part take your time and kill one enemy at a time. The Crossbow can be pretty useful to kill the guard at a distance.
Now you'll have to remove your wanted level, and the easiest way to remove it is to use a Pimp-A-Horse.