Here's an effective drill for developing a backhand slice from a defensive position for one player and for the other player to play more offensive. To start the drill the pro feeds a wide, deep shot to the player on the baseline (X1) who has to slice a backhand crosscourt. On the pro's command the other player (X2) starts to move in and must play a volley down the line. The point is played out between the players and players rotate after a certain time.
Variation: Move the starting point for X2.