For the most easiest way you have to unlock a certain character. The police man!

How to unlock the cop: Finish the main story levels and you'll be able to play the police man. ;) (source: Unlocking every character By Evil Resident)

He is crucial to make this possible.
He also prevents you to accidentally press the spacebar and use a trap.


I recommend you to take a map with the fewest people to kill.
e.g.:

• BikerParty (38 People)
  
  
• PoolParty (41 People)
  
  
• BoatParty (39 People)
  

NOTE: I don't think it is possible to get achievement progress by playing the Partybus!

The character has some nice and helpful advantages.

• He may carry dead bodies with no excitement.
  
• He can mark people as murderer by using his use-Item (F)
  
• He cannot use traps.

With the knowledge you have you're now able to start.

1. Search a person who is alone.
   
2. Murder him.
   
3. Get as far as possible away from the place where the policeman will come. (Stay close to a group)
   
4. Wait till someone sees the corpse and calls the police.
   
5. When the policeman arrives, mark someone of the group as murder.
   
6. Watch how the policeman knocks him down.
   
7. Wait till he is close to his car or whatever. (This should be enough time to recharge the use-Item befor the policeman leaves)
   
8. Mark the next one as murder.
   
9. Continue the steps 6 - 8 until the policeman "escapes", or all were knock out of the policeman.
   
10. If there are still to many to kill them alone, restart at step 1 otherwise kill them.

NOTE: Using the use-Item isn't a trap!

It may takes some time, but it's easy and there is fairly no risk to get caught. ;)