Dont know if that was already mentioned by others. The stun from enemys always negates your own attacks. It happens often when you charge with the warrior into an enemy, your are getting stunned imediately (even before the enemy's attack animation is shown) and your attack is negated. The same goes for all attack skills. For example you do Whirwind, the attack animation starts, and before you do damage you get stunned and your attack is interrupted, even before the enemys attack animation is shown. And you can also get stunned when the enemy is focused on the companion, even when you're behind the enemy.