The best place I have found to get this trophy is in Chapter 6 - Desperate Times.
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 so chuck a grenade over and kill all of them, then lower the small bridge to cross over to the rooftop where they were. Pick up the M32's and jump up to the higher rooftop here (there is a treasure up there, as well).
Now look left and down to another rooftop and you'll see four guys grouped together down there, so kill them with your nifty new weapon.
Now just reload the last checkpoint and you'll be back on the original rooftop you started on, so kill the dudes, pick up the M32s, and do it again. Rinse and repeat until you get your 50 kills and your trophy.Note: This is also an excellent place to get your kills for the Mk-NDI grenades, so see my other solution 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.