[Olorin]

I have to second Russ's vote for the best weapon. The Tuigan bow gives you more shots per round which means more chances to deliver your poison ability. Sneak through to the back of an enemy group and use your bow to disable all the spellcasters while everyone else comes in from the front. Keep arrows of dispelling handy for annoying stoneskins, and for tougher foes, hit them with the double poison: arrows of biting + poisoned weapon.

For melee attacks, the most important thing is that you use a weapon that's reliable. You're going to be using poison weapon for all the really important backstabs, so get a weapon that you're going to hit with almost every time. If you backstab someone and they don't die, remember that you can just leave, let your poison finish the job for you. This works especially well for spellcasters. If you move away, they will try to cast something at you, which will fail because of the constant poison damage. If you stayed around to fight, a good cleric might get a couple good hits in on you before falling dead from the poison.