For this achievement you need to kill 30 enemies with the XM25. This gun is only available in the first mission 'Black Tuesday'.
I read on TrueAchievements that you don't actually have to shoot 30 enemies with the XM25, you can just knife them. But for it to work however, you need to hold the XM25 in your hands.
You can combine this with just shooting with the XM25; for instance, you shoot 8 enemies with the XM25, and knife 22 while simply holding the gun the trophy will still pop.
You can combine this with the 'Jack the ripper' trophy, which is knifing 5 enemies in a row, so without shooting in between.
I haven't tried it myself yet, but I will in a few days, I will update if this still works.
I tried it today and knifing while holding the XM25 counts as getting a kill with the XM25, which means that this method (still) works