This involves timing a block with at just the right moment to stun a zombie, sending them reeling backwards. You’ll know when you’ve done this properly to standard zombies, as a button prompt will appear for grabbing and pummelling them up close. For Apex Variant zombies, a defensive counter will still knock them back and temporarily daze them, so mastering this is actually pretty crucial. You’ll probably end up doing this hundreds of times, so 25 isn’t really a tall order.