Kill 1,000 gas zombies

Starting in Case 6-2

Fortune City does its best Silent Hill impression, flooding with a thick green gas that makes some of the zombies significantly more powerful and gives them a projectile vomit attack. They'll be distinctively different than the normal zombies, wearing no shirt and looking like their head is half caved in. Re-dead 1000 of these gassy corpses and the trophy/achievement is yours.

The easiest place to accomplish this is in case 7-2 when you're going to the underground lab. Here, there will be hundreds of gas zombies ignoring you and clumping up against the door to the lab. Wade in with your favorite weapon and rack up the kills. Better yet, bring a Tesla Ball down, and do some real damage. When you run out of zombies, climb up the ladder just west of the lab entrance to leave the area. Build another Tesla Ball and bring it down, and you can repeat this until you're done.

Of course, if you have the SUV key, you can just drive up and down the strip until it pops.