Here's an extremely fast way for Mickey and Mallory to be found.
 

Step 1 Set the Difficulty to Be Easy

Main Menu – Option – Difficulty: 1 (easy).
 
 

 
 
 

Step 2 Start Gauntlet Mode

Main Menu: Start – Single player – Challenges
 
 

 
 
 

Step 3 – Play Until You Reach Level "Have a Blast", which is Level 4.

 
WARNING: Do NOT let time run out! Also, save your level as soon as it is loaded in.
 
 

 
 
 

Step 4; Get Molotovs & Go Crazy!

Throw molotovs to the crowd until it is five to ten seconds. Once this happens, QUICKLY press the suicide key (K by default) to die, then press the rejuvenate key (Space by default) to restart the level. After you load back into the game, save it again (preferably multiple slots).
 
 

 
 
 

Step 5 Repeat Step 4 Until Achievement Unlocks

This will net you about 300 kills per thirty seconds. This adds up over 15-20 minutes if you are able to kill 1000 people with this method.
 
 
 

Congratulations on your achievement!

 
 
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