Be ready for quickened spells for double whammy 1 round damage/death and time stop shenanigans. Even if he appears weak at the start, next round he could be godly. Backup plans for backup plans. Contingencies, blocking teleportation effects, gaining mindblank for your party, and so on, will greatly improve your chances. Getting into a spell off is unadvisable, though anti-magic field might be good if you can trap him in it and not yourselves. Basically, you want to lock him down and beat him with pointy sticks. Oh, and even if you do all that, he could just be an astral projection or a simulacrum of himself and it was all for nothing. Plus, he could have clones ready. So, expect to have to fight him 3 or 4 times at full health, and surprise nukes to the face after you think you've won. That is more dependent on how your GM runs him, though. But, that's what high levels casters can do.
Is it too late to play a dwarf?
Hardy, Steel Soul and Glory of Old will give you +5 on all saves. Seems pretty useful for when facing a wizard.
Lose out on 1 feat, 12 skill points and rather than floating stat you get +2 Wis/Con and -2 Cha.
Seems like a decent trade given that you are going anti-caster.
Also now that I think about it...
http://www.d20pfsrd.com/classes/hybrid-classes/slayer/archetypes/paizo---sl ayer-archetypes/witch-killer-slayer-archetype
Studied Target gives you a further +3 on all saves. And you get to take superstitious without the penalties. That is another +5 on all saves.
So you'd walk in there with +13 to all saves. And at Lvl 12 your class gives you +8/+8/+4.
Tag on Wis 14 (only 2 points), iron will (+2 will) and a cloak of resistance +3. That's a will save of +24. Also take clear mind so you can re-roll a failed save. Also assuming Dex16 and Con16, you have +27 to Fort and Ref as well. Tag on eater of magic so you can re-roll those as well.
Walk in, laugh at the wizard's magics, and use lingering thirst to start making his life difficult.
If you can get to 15th level the Reflection arcana gives you another layer of defence.
There's all kind of tricks a high level sorc/wiz might use. Mind Blank and other save boosters/reroll generators to make the mesmerist|fey sorcerer mostly useless is an easy one, Moment of Prescience & Foresight/ring of continuation to make sure he doesn't get nuked first round, Simulacrum to surround himself with utterly loyal copies - make sure you're really fighting Razmir and not copy #11. Also if he becomes aware of you he might make copies of your party, watch out for substitutions/infiltrators. Create Demiplane to ensure a perfect environment for him. The list goes on. If at all possible you're going to want to narrow that list down to what he actually has done and not what he could have done before you meet him.
BTW, Hammer the Gap adds very little damage. Maybe - assuming you're crit-fishing as a magus anyway - try Blighted Critical instead? Or Banishing Critical to cut down on summoned monsters.
Hammer the gap requires consecutive hits against one target in a single full attack action. If you hit four times, miss once, hit three more times then miss the last one (for example) then the damage bonus is 0/+1/+2/+3 and 0/+1/+2 = +9, doubled with the stamina combat trick (which I admit I missed before) for a total of +18 damage. It could be worse and wouldn't usually be the +42 of your example above. Miss chances make hitting 7/9 times unlikely too.
Elemental body or iron body are spells which give critical immunity and there's likely others. So, bad idea, sorry I mentioned it.
I don't think you'll get swift action movement for less than a feat's cost unless you go 3rd party or to D&D items. Force Hook Charge/spell combat moves you a little further than bladed dash though, 60' at caster level 14-15.
Arcane scent would pick up the real Razmir at least as much as any simulacrum. Sense Motive might well work if you can interact with them at all.
Max initiative and UMD. Scroll of Excellent Enclosure/Antimagic Field/Spellbane anyone?
Magus makes for a very efficient mage-killer.
Consider:
Plus the ability to cherry-pick the wizard spell list. Consider Echo Location, it defeats Mind Blank + Invisibility.
A kensai can easily have a 50+ initiative at this level, and Bladed Dash or Dimensional Agility ensure a full round attack on the opening round.
With Lingering Pain, you only need one good hit to prevent all spellcasting by your opponent. Phase Locking means even if he has a Contingency, he won't be going anywhere. Arcana Theft will allow you to steal any troublesome spells he may already have in place. True Sight means you won't be fooled by Simulacrum.