You used a "Move at angle action". This will complete instantly. Instead, use the bullet behavior actions "Set angle of motion" and "set speed", with a timer for duration.
Since you're already using the bullet behavior for the enemies normal movement, you can either add a second bullet behavior to control the knockback, or disable the normal movement events while the enemy is getting knocked back. Again, the timer behavior will be useful here.
[quote:9d7aj68z]When the enemy sprite gets hit by the player, set the enemy bullet speed and angle to the desired values.
You can also utilize the timer behavior to determine when to end the movement. On hit, trigger the timer for the desired duration, and on timer complete set the enemy bullet speed back to 0.