Notice: These ideas are not my creation, I am just posting to help everyone.
First things first, you have to have downloaded AND enabled the content pack to obtain the achievements. To enable, got under options and there will be an option to enable content packs at the bottom, go in select the packs and you're good to go.
Keep trying until you get on the map "Sonic Electronic Ball Breakers".
You can see the map name and layout by pressing left on your D-Pad. You will notice a hallway with a circular room in the middle of it, accessible from the 2nd floor. Go into this hallway and hide out in the corners. Watch out for enemies spotting you and shooting projectiles, expecially grenades at you. If this happens just switch corners so that they can't fire on you and make sure to keep watch for new threats. All you have to do is sit and wait for the juggernaut to spawn and for the other enemies to kill it. When they do, there will be a bright flash and loud explosion and your achievement pops without the frustration that following the other two solutions entail... Believe me, I tried several times before I finally came across this on some old forums. If you have any questions, don't hesitate to post/message. Also, if you feel the need to vote negatively, please have the decency to let me know why, so that I can update the guide.