At the end of Thugtown, after the propeller chase, is the area with the TNT and lots of zombies.
If you're looking to grind out some quick achievements, this is a great place for farming a lot of zombie kills, chainsaw kills, and cannolis since there are 3 of them in this little area. I got all of these in this area.
If you set off the TNT and wait too long, you'll get a checkpoint, and the big zombies will come out. If you restart the checkpoint after getting a fair number of kills, you can really rack up the zombie kills in a hurry.