If you haven't unlocked this trophy by the time you reach Chapter 6 - Desperate Times, then this is an excellent place to get it.
Toward the end of the chapter, you will be on a rooftop and you will be attacked by four or five guys on the rooftop ahead of you (Two of them are armed with M32's). There are some propane tanks lying around over there, as well, so wait for them to jump down then chuck a grenade over and kill all of them.
If you don't manage to get the trophy right then and there, don't despair. Move forward and lower the bridge so that you can cross over to the rooftop where you just killed those guys. Once across, jump up to the higher rooftop here (there is also a treasure up here) and look down and left. There will be four guys down there all nicely grouped together, so just toss a grenade down there in their midst.
If you still fail to get it, simply reload the last checkpoint and you will be on the original rooftop, so try it again.
Note: This is also an excellent place to get your kills for the M32 weapon and the Mk-NDI Grenades, so see my other solutions for that:It's most efficient to combine the Mk-NDI trophy and the M32 trophy at the same time, so check the other solution and get both trophies at the same time.