This will be the first mission you encounter Jackals (shield wielding enemies on). Overall, taking them down is pretty straight forward - either use grenades or rush them from the front/ side to melee their shield down then kill them, or just blast them twice with the shotgun. There are about a dozen teams of two scattered throughout the map, and the most frustrating part about this achievement is that your teammates might knock the shield of an offscreen Jackal down and it will come back up before you even realize what happened. Thus you want to try and keep pace so your teammates are always moving and don't get distracted, and when you take down a Jackal's shield, make sure you chase him down (they'll run away) to finish him off.
Alternatively, if the Marines are constantly stealing your kills, you can use the blue explosive canisters near the start of the mission to teamkill your marines and then just bounce around the map on your own to assassinate the Commanders, ignoring the Jackals entirely. Thanks to Darkened Night for the tip.
Note: There's an active camouflage pickup near the Hornet at the beginning of the level which will make 1-shot blasting targets easier with the shotgun.