One is all I needG 20

*requires you and your partner to have Arena DLC*
*recommend for higher level characters to do this. My partner and I did this at level 51*

Go to Arena A (easy difficulty) and kill everyone until wave 4. By wave 4, the infected start spawning. Have your partner cautiously kill all the Walkers and whenever an infected chases your partner, have him lead the infected to you, and that's when you kill him. Repeat this four more times. Make sure you do NOT kill any Walkers in between your streak. If a Walker comes towards you, run away! Let your partner take care of it.