Archery and rapid shots are a must. Precision shots and Massive crits are next up for increasing damage, but you may want to max evasion first.
Agi is your best friend, but you will want at least 3 str for massive crits, and 1 int/awa for precision.
After that, try to make sure your awareness stays at an even number for the damage boost.
The nice thing about archers is that you can max out their damage potential fairly early. From there, its all choice - do you want to kick and slow them down to keep on shooting? do you want to land explosive traps prior to the shootout? Do you want to stealth in and/or stealth out?
If you plan on sticking with the Archer aspect, then 7th house is your best bet. If you intend to respec later to a melee build then its a toss up between warrior guild and 7th house depending on if you want sustained or burst damage respectively.