It takes a bit of practice, but smoothness comes from not breaking a line until it's finished. It also takes a steady hand and a clean sketch so you know exactly where your line should be going. Sometimes my lines go a bit wobbly and not where I want them to though x_x Shading with pencils, I often use up to 14 different colours in total in my drawings. The sharper the pencil, the smoother the shading, though it does take a bit of practice not to press to hard with a super sharp pencil.
Well...I don't know about you, but I love that they seemingly based the "XYZ" Legendary Trio on creatures from Norse mythology.
Zygarde seems to play a somewhat-similar role to the Jörmungandr (or Midgarðsormr), also known as the "World Serpent".
I can see it being based on that...but Bulbapedia also claims it to possibly be based on Níðhöggr as well, at least in design. Are there any roots from a "Yggdrasil" (World Tree) for Zygarde to munch on, though?
That aside, I'd like to see its role expanded upon more. Whether it's in a possible "Pokemon Z" version, in an anime episode, or (more likely) in a future Pokemon movie...this Zygarde, the Order Pokemon, has definitely caught my interest!
The shading is really interesting - I love how there are subtly different colors all around the green underbelly to give it a scaly and shiny look. The dramatic shading, particularly on the black part, looks very cool and intense, too.