First off, thanks for the critique.

Alright did a major cleanup of the wording of the abilities so most of your questions about wording should be clarified. In addition I added some things to make Intelligence useful earlier in a Mage Slayers advancement.

In regards to your comments not regarding wording:

The class is definitely supposed to be similar to a rogue. Knowledge (the planes) makes some sense to me, enough that I'm willing to add it to the list. Knowledge (local) makes a bit less sense to me though, I see this as traveling too much hunting down magic users to really learn about a specific area.

I don't want the trapfinding ability to come at level 1 for a few reasons. First I really don't want this to be a dip class really and if it gave trapfinding in addition to it's other first level abilities I saw that as very dippable. Second, traps have to be above a 20 DC for them to need trapfinding, most of the traps that I've used as a DM and had to face as a player did not have a DC that high at level 1, or even level 3. Third, I think the Mage Slayer is too busy learning how to kill mages to get good at traps by level 1.

Sniper is not precision damage. Precision damage doesn't get added to all attacks, just the first one. That would make this a lot less nice. Mirror Image... not sure how I want to deal with that. I had originally envisioned the ability as similar to precise shot and improved precise shot, so I don't think there actually being more targets would qualify, but I'm already giving her the ability to see through similar illusions. I think I'll let them see through it.

I do want pincushion to be affecting nonmagical flight. The dragon thing did not cross my mind though, and I seriously doubt that an arrow would be able to drop a dragon from the sky, that would definitely strain my suspension of disbelief. I think I'll toss in a line that grants a bonus to the save if you're of a larger size.

The death effect thing, while I can understand the concern, is really needed. Save or die spells start coming at you around that level frequently, and without good fort saves, she doesn't stand a chance. I had originally thought of just making the Enchantment portion immunity to the charm and compulsion subschools. Would that be better, or should I give a bonus to saves against Enchantment, and if so, how much?

Dispelling Shot (Greater) is given at level 11, right when they exceed the cap. However, it costs more uses of Dispelling Shot, which depending on your DM and how many you actually have, could get problematic.

Improved Pincushion is supposed to be situationally strong.

What exactly do you mean by Stripping Arrow is dangerous? Do you think it is overpowered? For each use of Dispelling Shot I drain your highest level uncast spell and deal a very small amount of damage, which can be decreased further by a Fortitude save. Similar to what a negative level does, which can be done by a spellcaster casting Ennervate.

Mage Slayer does more than just give the arrow of death ability. It grants additional uses of Dispelling Shot and increases the cap of Stripping Arrow. It also gives her more DR bypass capabilities. The arrow of death thing is mostly just thematic, I fully expect that it will come up very infrequently. I see it as the epitome of an assassination though, so it had to be there.

Are the feats not granting enough for them to be being taken? I had originally given slightly bigger numbers for each feat but thought that they needed to be toned down after some playtesting.
For example, lets say we're facing a really powerful mage and we have Charged Dispelling, but only one time. The mage has the following buffs up. Mind Blank (lasts all day), Foresight, Blur, Mage Armor, Protection from Good, and Shield (all but Mind Blank cast when we tripped his Alarm spell).
I get a look at him, fire a Dispelling Strike (Superior) and use Charged Dispelling. He takes 22d4 damage just from Charged Dispelling. That's not counting my bow, my arrows, my Dex (from sniper), my Str (mighty composite bow), or Disruptive Strike damage. He has d4 Hit Dice too, so if he doesn't have a good Constitution modifier he's dead, one shot. Now he could theoretically make that reflex save and only take 11d4 damage, but that would still most likely take out half of his HP's.

I think that you're underestimating the classes capabilities. Against noncasters most of its abilities don't apply, but you could still use the Dispelling Shot stuff against targets that have been buffed up by someone else. Pincushion is useful against flyers, especially if they're using magical flight. You also get 7 archery feats, which is almost all of the core archery feats.

What would you suggest would make this more effective? I'm not sure that it's ineffective, but I also haven't playtested it extensively. What problems do you foresee it having?