A few comments:
--I'm not quite sold on Sitting Duck's ability to ignore things like Wind Wall, at least not from the start. That seems kinda strong for so early. Then again, I'm not very familiar with Wind Wall so I don't know.

--Arcing Shot seems like it could be moved up earlier in the class - that seems like something pretty simple that someone dedicating themselves to archery would learn pretty soon in their career. But once again, I'm not sure how powerful that is, so maybe it fits there best power-wise.

--If the archer can use Piercing Shot because he's good at finding chinks in the armor, perhaps a restriction involving studying them - i.e., they have to be subject to his Sitting Duck ability - before he gains the bonus against them is in order.
Or, maybe, at 11th level they have to Sitting Duck them to get the bonus; at 15th level they get it automatically against enemies within 30' and have to Sitting Duck those out to 60'.

Overall, I really like this class, good job on it. My comments should be taken with a grain of salt; people more experienced than I haven't seemed to raise these problems, so they may well be invalid.