This trophy requires you to build the Rocket Shield in Shadows of Evil and to then burst through a minimum of 10 zombies.
To get all the Rocket Shield parts you must first open the 3 districts and open up the areas containing Juggernog, Speed Cola and Double Tap. You can find all 3 parts laying up against walls or on chairs, etc. within the immediate area.
Once you have all 3 parts, find a workbench and build the shield.
It would appear you can only kill a specific amount of zombies with the shield under normal circumstances because I first attempted this on round 9 to a train of zombies and barely killed 4 or 5.
The method that worked for me was to keep the shield on me and wait for a random InstaKill to drop. While the InstaKill was active I pulled out the Rocket Shield, turned around and burst into the group. I rocketed twice just to make sure.
This worked for me and I was on round 28, so its never too late in the game to attempt this trophy.
A few seconds after you decimate your train this trophy should pop, if not, try again.
Here is a video by PowerPyx showing the part locations and how to get the trophy