Alistair or either of the two-handers will do fine.

Turn on Rock Mastery for noticeable bonuses to almost every stat that matters to your build. Take Bard as your second specialization, solely for Song of Courage. Cunning-based rogues avoid this problem, and can solve their own attack rating problem by taking Duelist at level 7. Dex-based builds are easier at first, but by mid-game the lack of armor penetration becomes a real problem. In the long run, cunning is better for you than dex (though you need 34 dex anyway for good bows). The best bow for you is probably Mage's Eye (unless you have access to The Sorrows of Arlathan, or are willing to drive Owen to suicide for the sake of Far Song.) Make sure you read about its mechanics on the wiki, though, if you plan to use it.) With a good tank, you'll be shredding enemies on Nightmare by around level 15.

(Though Arrow of Slaying can be good if you use the Rules Fixpack. You won't be using a lot of talents, as all of them take too long to draw to be worth interrupting your stream of high-damage auto-attacks. Succeeding as an archer in DAO is all about increasing your attack speed and critical chance as much as possible.

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