Some ideas:
1) One of these bows: Doublecross Bow, Epic Doublecross Bow, or Slaver's Hand Crossbow. They all have Nightshade Venom (
http://ddowiki.com/page/Nightshade_Venom). When it hits, helplessness ensues and 150% damage right there.
2) If you don't have it, Improved Critical Ranged and Precision (in certain circumstances) will also help. Improved Critical you probably know how works, but Precision gives you a better chance to do more damage against fortified targets. They won't improve your "numbers" but they will add more of the higher numbers...
3) Seeker items like Bloodstone and the like, can add to your critical hit damage as can any weapon with a "burst" or "blast". Seeker adds both to the confirmation "to hit" for a critical hit and to damage (pre-multiplier). So a Seeker +6 item with a typical x2 critical bow will add an extra 12 points of damage per critical. With a Doublecross or Slaver's, that's x2 x1.5 or 18 points per critical hit in combo with a +6 seeker item.
4) Deception/Improved Deception/Bluff (from you or your dog) in combination with a Sneak attack weapon can add to your damage as well. Even without sneak attack damage from being a rogue or monk, you can still add the damage when it would be a sneak attack opportunity and bluff/deception lets you do that. Additionally, if you don't have aggro other ways, should also work.