To counter non-scrappy slacking i have been using this set:
Giratina @ Leftovers
~ Heart Swap
~ Destiny Bond
~ Dragon Tail
You can switch into slacking with complete safety, steal his belly drum, and phaze him away. Scrappy slacking can screw you over but it isn't that common compared to illusion slacking. Almost any physically defensive pokemon that resists Espeed such as, escavalier, skarm, rhydon, regirock/steel, or any of the psychic/steel types, can run a similar set and beat slacking. One benefit of using giratina as a slacking counter is that gira is a good switch in to both slacking and the shedinja it so often mimics.
I can fly like a bird, not in the sky.
Which can always swim and always dry.
Say goodbye at night and morning high,
and I'm part of you.
What am I?