This can be a hair tricky, but as you play through the game you will get the hang of attacks and be able to do it.
To successfully block when an enemy attacks you, your buddy, or both, there will be a silver circle that appears under you. When this shows, you press the X button and if timed correctly, you will block the attack. You have to block all attacks (and also at least 1) in a battle for this to count. I found the easiest to do this on are Zombies when their first attack is Zombie Bite (since it is a single attack), but Elves, Humans, and rats are pretty easy to get this on as well.
This is easiest way to do this is when fighting a single opponent, and only giving them once chance to attack - let them attack so you can block it, and then stun or sleep them. After that defeat them and the trophy will pop.