The wrecking ball is a usable item, of which recipe's found in chapter 1 as a part of the story. Once you can make it, from 3 rockbomb shards, an iron ingot, and a cord, you can start going for this trophy. I'd recommend saving before attempting to get it, as that way you don't have to make more if you mess up.
When ready, head to the Hammerhood Heights. Once there, unequip your weapon so that you're just using your fists. The goal here is to start getting the enemies to chase you by hitting them once. You will need to hit them and then move a bit farther away to get them to chase you. Try not to move too far away from them so you don't lose their aggro. Once you've amassed around 10-15 enemies, you can plant your wrecking ball down. I wasn't sure if you getting killed with them would give you the trophy, so just to be safe, I'd stick around for a second or two and then run away. That way you can get as many of them as possible without getting killed yourself. Once you kill at least 8 your trophy will pop immediately.