It needs you Kill 53,599 infected. Not a hard thing if you keep play this game for a long time.
But if you just want to collect trophy, than follow this 3 steps.

First, you need to download 3 mod from workshop:
Left 4 Lib
Left 4 Fun
Left 4 Fun Sample Mods


Second, survival mode, chose survival map Crane (basically you can do the same thing in many map but this one I think is best), and input the following command in to Chat box (not command box):
!l4f_admin_init
!settings mod l4f_meleeonly_nospecial
!incap all

It will restart the game, no special infected spawn and only melee weapon.


Third, go to the switch and start the match! Then, stand here
Just one step behind the switch.

Now, turn off your monitor and have a nice dream. You supposed to get the reward while you sleeping.