First things first, you have to identify the problem.
If his AC is too high, you could review his sheet and check if he didn't
calculate something wrong (it happens more often than you could believe).
If he built his character around that much defense, he will have other flaws, but lets take a look at the information you gave us:
> For a short comparison: Knights AC: 28 (Full Plate-Armor, Shield and a Ring of Protection) Knight gets two attacks a round, 1d20+12 and 1d20+6, dealing 1d8+2 damage
>
> The Rest of the party has an average AC of 15. Attacks are ranging around 1d20+4.
Well, even though his AC is so high, his offensive power is that of a lv1 character (or lower), he will hit easily, but that damage is too low for a character of his level (7th).
His AC is actually 26, +9 from full plate armor, +3 from heavy shield (+2 base, +1 from shield focus), +2 from ring of protection, +1 from evasion, and only +1 from his dex modifier (it's limited by his armor).
Then, look for options that will "break" his strength, without hurting the others too much. There are a few options for that, based on the [Knight class][1]:
Reflex and Will Saves
---------------------
The Knight's reflex and will saves are bad, and the class is a martial based on charisma (similar to a paladin, but with nothing to boost the saves), which means the character is vulnerable to spells and effects that target those saves.
The character could take two powers that boost his saves against fear and charm effects, but everything else means he is weak against it. Illusions, necromancy, create pit, fireballs, entangle, web, etc.
Spells that grant negative conditions, such as [shaken][2], [nauseated][3], [entangled][4], [exhausted][5], or [sickened][6], are all good options, as some of those will lower his strengths and allow enemies to have a better chance against him.
If you *really* want to see him rage from a failed save, make an enemy cast [Create Pit][7] on him. And watch him strip after a bunch of failed attempts at climbing up (remind him that he has a -8 penalty from armor).
Touch Attacks
-------------
Most spells will either allow a saving throw, or a [touch attack][8], or both.
Knowing this, you should know that touch attacks will be checks against the character *unarmored AC* (10+ dex bonus + dodge bonus + deflection modifiers usually), which is much lower than his *armored AC*.
There are lots of enemies that can use touch attacks, so a quick look on the bestiary looking specifically for that feature will give you many candidates for an encounter. A quick look on the [wizard/sorcerer][9] spell list should be enough to find good spells to throw at him. [Corrosive Touch][10], [Mudball][11], [Shocking Grasp][12], [Acid Arrow][13], [Scorching Ray][14] to name a few.
Creatures that use touch attacks should be balanced against the whole party, as only monks should have a high touch AC.
Flying Enemies
--------------
If he built a character around defense and melee attacks, a flying enemy will be way out of his reach for most of the time, unless someone on the party casts a [Fly spell][15] on him. But even with the spell, since he wears heavy armor, his 40 feet flying speed should be lower than most creatures.
Difficult Terrain
-----------------
As I mentioned, he wears heavy armor, which means that unless he takes a lot of levels on the Fighter class, his speed should be lower than average (20 feet or so). [Difficult terrain][16] should be a nightmare for his character, as he will move only 10 feet per move action.
There are tons of ways for the GM or the enemies to put him into that situation. Most forests, swamps, deserts and mountains are difficult terrain (most GM's ignore this). There are spells (like [Entangle][17] or [Web][18]) that will force the terrain to become difficult for a period.
Ranged Attackers
----------------
Again, his character is slow and built around melee attacks. So ranged attackers, specially if flying or above ground level, should be a pain for him to deal with. Even more if they are able to move and move faster than him (and they probably will).
Combat Maneuvers
----------------
[Combat Maneuvers][19] are an option, as his CMD should always be lower than his AC. But being a martial, his CMD is still fairly high compared to the rest of the group.
If you have a group of enemies and they have the opportunity to flank him (which also grants a bonus on the CMB check), one of them should attempt to trip or grapple him.
Trip will lower his AC (-4), limit his actions, and force him to take attacks of opportunity to stand back up.
While grapple will force him to not move towards other enemies (like their boss which happens to be a spellcaster or ranged attacker), it will also reduce his AC (-4). If he is tripped and then grappled, his AC should be much lower now. Just keep in mind that, unless the attacker has reach or the proper feat, they will take attacks of opportunity as well. But since his damage is low, they should probably be able to take a hit or two and still trip/grapple him.
Aid Another
-----------
[Aid Another][20] is the simplest action that a henchman can take that will help his side to bring down a stronger enemy (the knight). This will allow them to make an easy check (DC 10) using their attack bonus to help another character to succeed at something.
This can be used on attack rolls, raise the AC of their boss, help tripping or grappling an enemy, and whatever else you can think of.
Encounter Advice
-------------
The cultists you linked have sneak attack and the [command][21] spell. I know that their sheet says that they will only use command on emergencies, but a towering enemy wearing a full plate armor should probably be a higher emergency than saving it to escape. Commanding him to drop prone will help them to grapple and probably pin him in maybe two rounds. You will have to consider their intelligence (10), or if their boss could come up with that idea on his own (14 int, probably yes) and tell his henchmen to do it.
For their boss, his AC is actually on par with your knight. His [suggestion][22] should be used as soon as possible on the biggest and dumbest enemy he can see charging at his beloved cult, while his *invisibility* is a terrible idea to use at the start of combat (despite what the developer said), as most of his other options are offensive and will be wasted casting early on. Instead, he should use his [copycat][23] as often as possible (he has 5 uses and they last 6 rounds), to prevent attacks from hitting him. On top of that, his [Reaper's Mask][24] allows him to cast the [Confusion spell][25] twice per day, with a DC of 16 (minimum to cast the spell).
Let me add that whoever wrote those cultists did a terrible job at it, their feat selection is awful. Selective channel with -1 charisma. The feat does not say it affects at least one target, meaning that it does nothing at all. They took martial weapon proficiency simply to use war razors (they get +2 to conceal it) instead of using daggers or short swords (their deity's weapon) and take Skill Focus (Sleight of Hand). And why on earth would he take Weapon Finesse if he already got Finesse Rogue? I suggest you replace at least two of those three bad choices with teamwork feats, such as [Precise Strike][26], or something that suits their sneaky murderer role, such as the lines of feats for [Strangler][27] (3 feats total).
Again, all of these are simply hints on how to make combats difficult for him. Remember that the objective of the game is to have fun, so you create challenges that they must overcome. If you, as the *Game Master* start to play against your players, they will get annoyed or worse, and might not come back next session.
So don't abuse these extra challenges, make his character shine in combat once in a while and he will have fun with the character he built. Throw a [Stone Giant][28] against him so he can brag about it ("*I survived the blows of a giant*!").
PS: There is [a thread going on at paizo.com][29] exactly about the subject of how to use creatures against your players. It's worth to take a look for a few hints, lots of good advice there.
[1]: http://www.d20pfsrd.com/classes/3rd-party-classes/tripod-machine/knight
[2]: http://www.d20pfsrd.com/gamemastering/conditions#TOC-Shaken
[3]: http://www.d20pfsrd.com/gamemastering/conditions#TOC-Nauseated
[4]: http://www.d20pfsrd.com/gamemastering/conditions#TOC-Entangled
[5]: http://www.d20pfsrd.com/gamemastering/conditions#TOC-Exhausted
[6]: http://www.d20pfsrd.com/gamemastering/conditions#TOC-Sickened
[7]: http://www.d20pfsrd.com/magic/all-spells/c/create-pit
[8]: http://www.d20pfsrd.com/gamemastering/combat#TOC-Touch-Attacks
[9]: http://www.d20pfsrd.com/magic/spell-lists-and-domains/spell-lists---sorcerer-and-wizard
[10]: http://www.d20pfsrd.com/magic/all-spells/c/corrosive-touch
[11]: http://www.d20pfsrd.com/magic/all-spells/m/mudball
[12]: http://www.d20pfsrd.com/magic/all-spells/s/shocking-grasp
[13]: http://www.d20pfsrd.com/magic/all-spells/a/acid-arrow
[14]: http://www.d20pfsrd.com/magic/all-spells/s/scorching-ray
[15]: http://www.d20pfsrd.com/magic/all-spells/f/fly
[16]: http://www.d20pfsrd.com/gamemastering/exploration-movement#TOC-Table:-Hampered-Movement
[17]: http://www.d20pfsrd.com/magic/all-spells/e/entangle
[18]: http://www.d20pfsrd.com/magic/all-spells/w/web
[19]: http://www.d20pfsrd.com/gamemastering/combat#TOC-Combat-Maneuvers
[20]: http://www.d20pfsrd.com/gamemastering/combat#TOC-Aid-Another
[21]: http://www.d20pfsrd.com/magic/all-spells/c/command
[22]: http://www.d20pfsrd.com/magic/all-spells/s/suggestion
[23]: http://www.d20pfsrd.com/classes/core-classes/cleric/domains/paizo---domains/trickery-domain
[24]: http://www.d20pfsrd.com/magic-items/wondrous-items/wondrous-items/m-p/mask-reaper-s
[25]: http://www.d20pfsrd.com/magic/all-spells/c/confusion
[26]: http://www.d20pfsrd.com/feats/combat-feats/precise-strike-combat-teamwork
[27]: http://www.d20pfsrd.com/feats/combat-feats/strangler-combat
[28]: http://www.d20pfsrd.com/bestiary/monster-listings/humanoids/giants/giant-true/giant-stone
[29]: http://paizo.com/threads/rzs2u2xj?Why-are-published-monsters-not-working