I went through this the long way (legit), but when I hit the 86 kill mark, I kinda got trigger happy. If you find a Street Crime Case (SCC) that has a lot of 'bad guys' for you to kill, then I would suggest (for those who just want to get this over with) that you kill all but 1 and then let the last guy kill you, which will start you over on that SCC. The kills do carry over.
Just think that 100 is not that much and you'll do fine. ;)
Hope this helps!