1. Set up a lobby in Campaign Mode with Mutation: Chainsaw Massacre.
   
   
2. Select Campaign: Dead Center, Chapter 2: Streets and Normal difficulty.
   
   
3. Play until you reach the Gunshop section; it takes about 5 min.
   
   
4. Kill the bots so they don't steal your kills and medkits.
   
   
5. Follow the objective and open the store door, but don't pick up the Cola and simply camp behind the counter with your chainsaw.

The kill rate is about 50-60 per min (~3000-3600 per hour). There are no Special Infected in the store, so even if this is not the fastest method, it requires very little concentration, so you can chill and watch/listen to something else at the same time.

Of course, the least boring way is to just play with friends normally and unlock this over time, which is how I got over 80% of the required kills.

Occasionally, Hazmat Suit Infected will drop some Boomer Bile so you can use it to spawn an extra wave of zombies quicker.

If you have any questions, feel free to comment below.