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 wait for them to jump down, then chuck a grenade over and kill all of them.
Now just reload the last checkpoint and you will be back on the original rooftop, so chuck over a grenade and kill the dudes again. Rinse and repeat until you get your 30 kills.Note: This is also an excellent place to get your kills for the M32 weapon, 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.