Remember to focus on your footwork as well because slice balls can be very difficult to hit due to the bounce and skid so you need to prepare for your shots well.
Yeah just to add to this, you need to take your racquet back much earlier than usual to compensate for the low skid and bounce, especially if you intend on responding with a drive. It will feel unnatural at first, but the timing will come if you keep practicing. It's been especially tricky for me at the moment with the damp outdoor courts in the UK!
Also, although this may be against your ethic, I find responding with a slice of your own to be effective as this is what they least expect. If you try to pummel it back at them, you're playing into their hands sometimes as they like to be on the defensive with their slices, so maybe try changing it up a bit?