Sorry, but Sylvanas is not proving to be the dps some had hoped for. While she can be very impressive, this is usually just bullying based on level/stat leads. In sustained team fights the required space to teleport can be a difficult limiting factor when looking to spam the q ability. I honestly have found Sylvanas to be one of the best and most efficient mercenary capping champions in the game.

So the idea here is to be passive and neutralize creeps with shadow dagger and auto attacks until you get to lvl 7. With mercenary lord in hand you should proceed to the easy camps. Whichever siege giant does not have the purple target on it should be the target of your auto attacks while the withering fire stuns the other. Use your banshee to quickly refill your arrows. Once you are around lvl 8 you should be able to take on the bruisers but it is possible, while more difficult, to go straight from easy to hard camp caps solo. The key is tracking which mercs have the purple target which symbolizes what the q ability will target.

So once you have double arrows and vulnerable you can obliterate camps which makes you very dangerous for stealing enemy camps on smaller maps with more exposed bruisers. Your major contribution to team fights is going to be your wailing arrow which should usually be targeted on the main healer or dps ( but preferable you would like some other characters in that blast as well ). With the enemy team silenced and vulnerable from your dagger, you should usually back away and auto attack until the need to chase comes which is when your withering fire comes in handy for getting the takedowns.