You need a shield with "Parry" skill. When a boss attacks you, you need the press the button just the right time. It is one of the hardest thing to do in a Soul game actually. at least for me. You have to hit the Parry just miliseconds before the opponent's attack hits you. If you success then you will hear a sound then boss will be vulnerable. just do an immediate attack to boss for a critical hit.

It takes a lot of practice. Just watch videos so you can understand better.