Dexterity would be fine, except it then excludes a Wisdom based archer who uses the Zen Archery Feat. So, for instance, a Cleric/Sorcerer who went Arcane Archer would then be cheated. A purely level based bonus to the DC would be the best way to go with something like +1 to the DC every 2 or 3 levels of AA, in my opinion.
Maybe base dc of 20 plus 1/2 AA levels.
How many times a day does the AA get an "Arrow of Death"?
If it makes the DC save, let 'em bleed until healed by Kit/spell/potion.
I still think that having a set dc modified by an ability bonus would be best.
Then 20+1 per AA level seems appropriate.
Don't forget that if it's changed then PC's will have watch out for those AA's that attack us. Wondering if deathward, shadowshield or foe protect agaist the AA's death arrow?