Does it have to necessarily be a necro archer? I find Vamp Form to be generally a waste on ranged fighters. Wraith form is great for mana leach... but the slow movement can impede you in the event of being swarmed by mobs or a raid. Are you also using necro for other casting abilities?
While I now generally prefer my Sampire for any spawns, I previously would use ABC archer on Lord Oaks and do generally okay there. Granted, we were usually in a group of atleast 4-5. I would also suggest different bows with different set ups to see what works best (i.e., different elemental bows with AI, bows with various hit spells, AOE effects). Once you get into a comfort zone with the different weapons, you can set your macros to be able to change with one button in the event you want to fire up at the ridges and then change back to different weapon for ground level or whatever else you're fighting.