Mission: The Gulag
Really simple this one, eventually during this mission you’ll go to the armoury and there’s a Riot Shield in there. Pick one up, once the doors open melee a few of the enemies with the Riot Shield and the achievement/trophy will unlock.
This can also be earned on No Russian
but that involves picking one up from an enemy. This can also be earned on the Just Like Old Times
mission as you'll need to grab a shield on there also.
If you're still struggling, feel free to use this as a reference.Call of Duty: Modern Warfare 2 Campaign Remastered - Unnecessary Roughness Achievement/Trophy Guide
Hope this helped.