Learning how to return a lefty slice serve is difficult.
One idea that will help is to try to play more against lefties who use the slice serve a lot. Getting the timing down is the hardest part.
You said that the serve was "junky, having little pace" and that you opponent has good movement.
The problem that you may be having is trying to do too much on your return. Although your opponent is fast you do not have to hit your return that hard or with a lot of spin for it to be effective.
All you have to do especially on a low skidding serve is to block it back preferably deep, that's it.
A more complex strategy is if you get a very short junky ball then push it deep and low down the line then charge the net. If you prefer to stay back then just push the ball high and deep to neutralize their serve.
As long as you get the serve back deep and consistently you will take your opponent's lefty slice serve out of the equation
