Disclaimer:
I'm not a pro by any means and this is just a first pass at tweaking what I feel is an awesome specialist character.
Through Sylvanas' passive, Black Arrows she has a HUGE amount of potential for not just shutting down minions and lanes but soloing mercs early on.

Even though I *love* the flavor and potential of Posession, especially it's extremely low cooldown, I felt like Wailing Arrow's silence and dmg made it the better choice. Even more so later in the game thanks to Deafening Blast

Anyways, this isn't an in depth guide by any means so sorry in advance if this isn't your cup o' tea. ^_^
GLFH!