In order to to hit an effective backhand slice there are some important technical elements that you need to be aware of:
Start with a Continental Grip. Use the non-dominant hand to help you take the racquet back. Use your torso to take your racquet back. Keep the racquet head open in the back. Step towards the ball and stay sideways through the swing. Use the non dominant hand for balance and as a counter movement to the dominant hand. Make contact with the ball while still sideways. Hit through the shot with a fairly level swing. Do not hit down or around.