If we're going the sneak attack route (which fits the concept best, IMO) then we definitely need the Sniper's Goggles, which allow us to make sneak attacks from any range rather than being restricted to 30 feet. There are other ways to increase the range limit, but nothing that gets us into the hundreds of feet which is what we're looking for. Now we can get our juicy sneak attack dice on our unaware targets from a ridiculous distance away. Sadly this eats the majority of character wealth at the 8th level, but it's completely necessary if you want sneak attacks at long range so you have to eat that cost (Edit: I just noticed we're talking about NPC's, so this is definitely outside of budget; you'll probably have to raise the CR in order to use PC build rules to do this gracefully)
I think the most straight forward build would be a Goblin Unchained Rogue 8. You could do a bit of multiclassing (be sure to buy back any lost sneak attack progression with the Accomplished Sneak Attacker feat) but that's strictly up to taste. With 8 skill ranks, +3 class skill, +4 racial, +4 stealth, and +6 from dexterity gives you a +25 stealth check. With skill unlock (stealth) you only take a -10 penalty when sniping, meaning you have a +15 stealth check to attack while keeping your position concealed, and that's before considering bonuses from distance.
If you want to go for the Wizard side, I'd say that the Arcane Trickster is your best approach. It gets you nearly full spellcasting and sneak attack progression at the cost of a terrible BAB. So you won't be very good with bow attacks, but your ray spells will be pretty deadly. Eldritch Scoundrel is also an option, but if you want decent sneak attack power it's going to be horrifically feat starved and that's a problem for an archery build.
Sniper archetype slayer, accomplished sneak attacker, the usual archery boosts, possibly a dip into ninja for more sneak attack and poison use.
A fun way to do this might be as a kineticist, aether into air specifically. Take the wild talents extra range, extreme range, and via the "extra wild talent feat" chosen at level 7 for air's reach. This lets you use air/electric blasts from 960 feet away. Meanwhile, take Telekinetic Invisibility at level 6, and the feat "conceal spell" This way, for 0 burn, you can hit someone with an air or electric blast from 960 feet away forcing a perception check to even notice that you did anything at all (which, from 960 feet away is a +96 to the DC for the party). In case you get made, you can use telekinetic invisibility to slink off, and reposition. If you can wait until level 10, you can do all this while flying.
Your damage for this is only 5d6 + change, but you can empower it for burn (assuming you don't want to gather power) and get a lot of shots off before the target can do much about it.
Sniper archetype slayer, accomplished sneak attacker, the usual archery boosts, possibly a dip into ninja for more sneak attack and poison use.
That works! Probably Slayer 3/Unchained Rogue 5 would work best, since that gets you skill unlock (stealth), which is a must-have for any sniping build (seriously, it turns a -20 penalty into a -10 penalty).
Love the kineticist option; I really like how kineticist builds like these tend to come online so cleanly before 10th level.