I found an easier way to get the Hot Potato achievment.
Start Up chapter 5.2 and as soon as the level starts lunge towards the Ghost he will then use his shotgun and make you hit a pipe on the wall. The pipe makes you catch on fire, after that you can lunge at non ghost enemies and make them catch on fire. Then just proceed till the game saves and you can load the level back up. You can easily get 9 enemies on fire using this method.