Smooth finish in Slic3r using Support Material
I'd say the clue is the material itself and the geometry of the object. But in terms of Slic3r settings you could experiment with:

(print settings >> support material)


pattern spacing (he has here something around 5mm)
pattern (I'd say the best for you is rectlinear)
contact z distance (choose 0.2 or close to that)
interface layers (here 0 (zero) for sure)
don't support bridges (this one depends on your geometry and if slic3r calculates something as a bridge or not)


All above is also connected (in a way) with HE temperature and layer height. Usualy the higher temperature (in a range for specific material) the better sticking between layers.