Start a game on easy. You will need to get 2380 power (money). There are three possible ways to get power.

The first way is by killing enemies:



The second way is by gathering valuables (Recommended):



The third way is by hiding behind civilians and let the mercenaries kill them:

Once you get 2380 power. You should buy Health Upgrade 1-4, HMG, Brute and Medkit.

Now you got everything you need to get the achievement!
Search for the shield-wielding Confederate. Once you find him, kill every other enemy except him. Use your medkit to regain health and move closer to him and make him move backward, then force him into a corner.

UPDATE: It is a better idea to lure him into a building and try to get him stuck in-between table and chairs or the corner of the building. Remember: moving backwards makes him come closer to you and bumping him makes him go backward.



Now that you got him stuck in a corner, you have a very limited time before he kills you. so you have to do it quickly. Take out your HMG, crouch and deploy (Make sure that its fully loaded). Focus your shooting on his shield!



After your magazine empties. You will get the achievement after a couple of seconds, however if you did not get the achievement then reload and try again.



I got the achievement on my first try.
If you need any help or have a suggestion then feel free to add me or comment.