No idea what a PowerPoint stalker is, and I'm not sure I want to know.
The problem with being a reach weapon fighter vs. mages is that there's 101 ways for the mage to get out of reach or otherwise avoid AoOs, only a few of which can be beaten by Step Up. It might be more effective to go for readied ranged attacks (from overwatch style) and a mount to move them around.
If your players have no idea about those 101 ways then sure, go for it. I'd recommend having snake style to maybe avoid one devastating attack per round and some potions of resist energy to mitigate magus-nova. Step up, following step, and step up and strike should be on their list of feats too. Disruptive is more or less required.
We could just build a Witch Killer Slayer. They seems to be best equipped to handle arcane casters. Another route is to use drow and their innate spell resistance or take the Accursed Feat.
As an alternative to the above Witch Killer Slayer, a core Barbarian with the Superstition, Witch Hunter and Spell Sunder rage powers creates a great anti-caster NPC. Unlike the Slayer, if you gain a means of rage cycling, you can Spell Sunder every round. A dip into Oracle for the Lame curse at 9th level, for example, on an NPC with 8 Barbarian levels works out to be a nice CR 8 for your 8th level party. With four of them in the room, that would be a CR 10 encounter.
DEX & INT focused Kensai Magus gets great initiative (capable of beating most casters), as well as great touch AC (good for dodging a lot of annoying spells, like Enervation) and a balanced set of saves (though it should be your priority to pump this up as much as possible).
More importantly, they can cast buffs like True Sight to negate most caster defenses, as well as Dimension Door to close distance. They can even do this while still making full attacks (YMMV if they need Dimensional Agility for the latter).
Barring this, Pouncing Spell Sunder Barbarians can be quite the terror to the unprepared spell caster. Even more so if they can sunder effects that can't otherwise be dispelled (like Walls of Force, for example).
Readied vital strikes with a heavy crossbow to interrupt spells will keep them on their toes plenty. It will definitely disrupt their fight rhythm and force a bit of creativity.
I would add some monk minions with good saves too function as fodder and defend the boss.
Eldritch guardian with a mauler familiar can be good if you do a dirty trick build.
When you have dirty trick master (needs BAB+11, as well as general maneuver feats), you can stack dirty trick conditions to make a worse condition. This can make sickened turn into nauseated- no standard action to cast spells, and no standard action to get rid of the dirty trick. This basically means that the affected character is useless for 1d4 rounds (even quickened spells don't do much at that level).
The Eldritch guard lets you share your combat feats with your familiar- it works best when you look for good action economy. Dirty trick master needs 2 dirty tricks to activate, which usually means two standard actions- which is great for this, since the familiar is an extra character.
So this fighter and his pet can zoom around the room, incapacitating an enemy every turn. This can even be used by the NPC to 'rough up' the party without actively threatening to kill them.
Dwarf Fighter, Shatterspell, Sunder Blessing, Abundant Tactics, Steel Soul.
There's some prerequisites, but this skeleton makes the anti mage. Invest in Sunder.
Arcane bloodline Bloodrager does surprisingly well, particularly with Improved Unarmed Strike, a reach weapon, and Enlarge Person.
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An Unchained Monk with the Dimensional Assault feat chain. Pick the prime fighting spot. The Monk can teleport in and out of the room or behind obstacles and they will never know where he's coming from.
Issue with this is that you could easily kill a single player so you may want to spread the attacks around.
This will keep them on there toes.
I had a vanilla monk with a custom magic necklace of anti magic field. All they gotta do is jump on and grapple nearly any mage.
I had a vanilla monk with a custom magic necklace of anti magic field. All they gotta do is jump on and grapple nearly any mage.
I've known a DM that uses anti-magic fields like this. That's not so much challenging players than it is rendering them useless with no chance to resist.