d6 hit die and 6+ skills suggest that it's a rogue but less so. 3/4 BAB reinforces that. Good Ref and Will is thematic. Skill list is pretty reasonable, might also want Knowledge(Local) and/or Knowledge (The Planes).

Point Blank Shot is on the table but not in the class description. It looks to be a bonus feat, throw in a line to that effect.

Disruptive Strike is bonus damage, which archery generally needs.

Evasion is, again, like a more specialized rogue.

Track makes sense. Trapfinding usually comes at the first level of a class, since some games start at level 1. This makes it easy to dip for trapfinding but rogue already lets you do that, so...

Does the Mage Slayer have to qualify for their bonus feats? I would assume so, but some bonus feats have wording that exempts them from pre-reqs. It doesn't hurt to specify here.

Dispelling Shot counterspell needs clarification. Does it automatically counter the spell if you hit the spellcaster? As it is currently written, I think the answer is yes.

Sniper... is the Dex to damage a precision bonus thing? Mirror image is an illusion that makes it harder to hit people, but it doesn't grant normal concealment. You might want a special line in there just for that spell.

Pincushion is weird to be an Ex effect, but such is D&D. You seem to be going for some MAD by adding a desire for Int, but this is pretty late in the class to start that. This also makes the Mage Slayer rather annoying against dragons, who have ~10 Dex and therefore could be dropped from the sky with a single arrow. Maybe this should only work against magical flight?

Guarded Mind and Body is very strong. Deathward is a 4th level spell that provides immunity to death effects for minutes/level. At 9th level you give part of the effect of the spell all day long. In addition, the character is now immune to an entire school of magic. Possibly too strong.

Greater Dispelling Shot is necessary at some point because Dispel Magic caps out at a +10 to dispel checks.

Improved Pincushion is situationally strong.

Stripping Arrow is dangerous.

Guarded Soul is SR.

Dispelling Strike Disjunction needs cleaning up. Is it single target like dispel magic or an area like Disjunction? Does it affect magic items like Disjunction or just active spells like Dispel Magic? I think the answers are 'single target' and 'effects only' but it's not entirely clear. Take out the reference to Disjunction and state that "for 4 uses of Dispelling Strike, a Mage Slayer may dispel all ongoing effects on a target without any dispel checks."

Mage Slayer is... it doesn't really slay a mage unless you have met said mage before.

The feats are mostly not really worth taking, with the possible exception of Superior Pincushion because it doubles all damage you do to targets in the air. Depending on the campaign that might be worth it. It's nice to have the options though.

As a set of abilities for killing spellcasters, this class is decent at shooting arrows at them and has a few useful defensive tricks. If enemy spellcasters are frequently in battles without pre-buffs this should do ok against them. It is very limited if it is not fighting spellcasters though... I would rather have a rogue than this class if we are raiding a bunch of orcs even if the orcs might have a shaman or two.