Diablo has the "hot-headed" trait. This means that he will occasionally overpower his attacks. This works just as if you scaled up the power of an attack, making them stronger but requiring more energy, and you have no control over it. However, if he tries to overpower an attack to the point where it takes more energy then you currently have, he will end up being stunned. Watch when you use his Hellfire, and stop him if he is going to overpower it.