Think like skimaru, Susano is a skeleton that defends the user whilst at the same time allows the user to send attack through it to his enemies. Therefore the defense opens, at least for an instant during the attack. You need to attack the susano whilst its user is attacking. However you need to attack with the correct element. Say madara does a fireball, then silmunateously whilst the fire is passing through the susano, water should be sent through it. This will neuralise the fire and pass water into the susano shield. Now this is where it getts interesting, you can infuse the water with lightning to cause damage to the susano user. You can infuse the water with mud or sticky substance that solidifies inside, which in turn traps the susano user.