State of Decay: Year-One

Once you've reached Breakdown Level 5, look for hordes and try to go BEHIND the horde, crouch (hold B) then sneak up behind one of the zombies, press and hold LB, then press Y (while holding LB) and you'll smash the zombie's head on the ground, if its done properly you wont get detected, reapeat this process on all other zombies of the horde and you'll get 7-8 easy kills, you shouldn't have much trouble finding hordes just remember to attack them from behind

Helpful tips:
If you have a character with Wits skill maxed out, then choose the Ninja skill specialization, this way you'll be able to move faster in stealth mode

An easy way to do this is to use a nimble character with the spin kick skill. The spin kick will knock zombies face down on the ground. You can then quicky crouch and hit LB+Y and the character will perform the stealth kill.

This can also work with powerhouses with pro-wrestling, but the zombies land face up when you throw them so you need to position yourself above their head so the zombies can't see you to trigger the stealth kill.