The first time the contingency saves you, but then you need to start worrying about if anyone got away to tell what contingency you set, which is another thing to worry about. If you just use the same "perfect" contingency all the time, someone's going to notice and use it against you. If you vary, you might not have the right one up when you need it.
Sure, someone studying you continuously might begin to see a trend, but I doubt it's as easy to determine as you seem think. From the characters' perspective, there is absolutely no difference between a contingencied teleportation spell going off (for example), and the spellcaster casting a silent teleport spell (or other spell) immediately prior to a killing blow (or other trigger).
One of my friends is running a Two-Handed Ranger / Shadow Dancer build that has absolutely murdered any mage we come across. Shadow jump through the shadow of a nearby party member and out of the shadow of the mage, ready a standard action vital strike for when mage take their turn, mage now has to make DC 10+4d6+25 concentration check (at least). Rinse and repeat.Reading this, I presume mages and wizards in your game do not usually fly in combat!
Antilife Shell + Overland Flight FTW
Also, mages is your world live in dim light? Have none of them ever bothered with a few Continual Light spells so they don't strain their eyes reading all those arcane texts?
War Priests with Vital Strike and Greatsword Battler Divine Fighting Technique, plus Improved Critical and an 18-20/x2 weapon with Greater Bull Rush and Bull Rush Strike can be really nasty vs. casty types. Step Up & Strike is also really nasty.
Greatsword Battler allows you to make a Vital Strike on a charge, and allows you to make your Vital Strike dmg count as continuous damage for concentration checks at lvl10. They can spellcraft and counterspell with dispel too.
It's really hard for them to cast when they're getting critically hit at 30% chance and bull rushed everywhere, and even if they do get a cast off, Dispel and keep nomnomnoming. War Priest action economy is insanely good too, you can heal damage on the fly and still get full round attacks. If he gets away from you, charge and Vital Strike.
Get some good SR gear and you're golden.
Huh!
I feel obligated to pitch in.
While there are many strategies (none of which can hold up at the really high levels), my favorite is ranged vital strike, kind've like people mentioned. If you grab a bow or crossbow, and then ready a vital strike against spellcasting it can really interfere with them.
This works best with ambush tactics pre-buff, but still pretty applicable. There is one feat that I believe gives you a bonus to distraction for readying ranged attacks, and I know overwatch style can give you multiple ranged attacks.
Arcanist with the School Savant archetype. Divination as the school of choice. This gives you that nice bump to initiative and prevents surprises. Also divination is most likely to remember to stop others from divining them.
Exploits:
Dimensional Slide - to keep you from being pinned Down
Counter Spell - Game changer since you don't have to prep dispel magic all the time or waste your turn with it as a trigger action to counterspell
Spell Disruption - take apart your enemies magical defenses
Obfuscated Spell Casting - in case they also enjoy counterspelling
Spell Resistance - self-explanatory
Greater Exploits:
Counter Drain - Recharge your arcane reservoir when you counter spell
Dimensional Seal - Keep others from teleporting away from you and your allies
Greater Counter Spell - Easier to counter others
Greater Spell Resistence - higher SR
Greater Spell Disruption - Easier to dispel effects
Resistance Drain - recharge arcane reservoir
Siphon Spell - recharge arcane reservoir
With the archetype this will take 5 feats
Wizard->EK with a trebuche. You have an absurd range of 8000ft aka a -800 on the perception check to avoid a surprise round. Not only that but they require teleport to even start combat with you. Between vital strike, special ammunition, and energy siege shot they should die on the first hit. Since its a static dc knowledge engineering check that you have multiple options of spells for rerolling your not missing a single shot. Your still a wizard with full caster level so finding their clone should be at least easier than anyone else. Worst case scenario the shot is deflected by plot armor or the squishy mage somehow lives though the 25 or more d6 of damage(1st shot) and you are given a full round to run away
There is a lot of martial vs casty debate, with a lot of hate on the casties, because at the end of the power curve casties win. But there is no particular trick to beating a casty at low to moderate levels.
Win initiave. Win.
It doesn't matter if its an archer, a pouncing barbarian. Whatever.
Up to level 9 or so, martials win.
After level 9, its starts to switchover. But this is why the worlds hardest dungeon - the cheesegrinder - is almost always won by casty types.
Secondly, the ability of am barbarian to win is largely fictitious. Casties win by action economy.
Something like: Wizard casts dc 35 quickened dazing magic missile on barbarians mount. Laughs as barbarian goes no where.
Makes a readied action on barbarian to move 10 feet. Builds a force cube (or stone cube) around barbarian. Laughs as barbarian loses turn.
Familiar uses a wand to summon earth elementals.
And even if by somechance all that were to fail, the wizard will still have
permanent mirror images, permanent wind wall. Emergency force sphere as an immediate action, and contingency teleport away if attacked.
And while a tertori monk is great, no it does not have all the answers for mage killing either. Whats the tertori going to do when the mage (and familiar) summons 12 lanter archons and then leaves?
Vital Strike Archer: Emergency force sphere.
Wow, a 5 year necro. That said, I personally like using a Barbarian with Greatsword Battler, Greater Vital Strike & Furious Finish.
The advanced benefit of Greatsword Battler makes your damage count as continuous for the purpose of concentration checks, making it easy to finish off any opponent you manage to use vital strike on.
Then you have Spell Sunder to negate caster buffs and debuffs, great saves through Superstition and Eater of Magic, and if you've really gone overboard on rage powers, you could even get Ultimate Clarity, which is basically a true seeing that doesn't get blocked by Mind Blank.
At level 20, you'll probably want the Superstitious Archetype, which gets rid of DR for scaling senses and initiative bonuses. Blindsight is pretty difficult to fool.
That said, you still don't have a particularly good chance at taking down an optimized mage, but you definitely stand a better chance than most.