The key to playing Lunara is to realize she's a specialist, not an assassin.
First off, she doesn't have the speed to consistently join teamfights in a timely fashion. Second, her 200% increase to poison damage applies to structures, as well. Third, you can spam her 'W' ability while shifting targets to blitz a minion wave very early on. Third, she has vision to help prevent ganks when going ham in a lane.
She's not ideal as a replacement for Raynor, but if you build outside the box, she can consistently push 300,000 to 400,000 Siege damage.
