As an adult, go to Hyrule Market and kill a zombie to make the other zombies move towards you. hit one of the other zombies until there is only one hit remaining until it is killed. Then, try to get another zombie to get close to the one you are about to kill. They should be close enough where you can hit both zombies. When this happens, hit both zombies with one sword attack. As one zombie dies, the other one freezes. This is easier to do with the Big Goron's Sword.