Wizards' options against...

[Dr. Azkur]

I'm not that savvy on the Wizard, so I'll ask:
What are the options a Wizard has to prevent being defeated by means of Momentary Disjunction (Witch Slayer 5)?

(For the purposes of this exercise a mage killer übercharger charges at the Wizard the same turn he causes the disjunction).

Present two cases, one in which the Wizard is alert about someone coming to kill him (in this case also provide one way he could have found out, and how much he would know) and one in which he is unaware. In both cases state the level and basic orientation of your wizard (Generalist/Specialist, relevant feats and variants, etc).

Thanks in advance!

[(Un)Inspired]

The problem is that I wizard will never be caught unaware. A contingent spell set to cast scry the second anyone is plotting against him lets him see both them and all their plans.

Second, A wizard should never be failing any saving throws (this is being generous and assuming that the wizard is willing to condescend to let a witchslayer zap him). With spells like moment of prescience a wizard can make basically any save even on a 1.

Lets say that a wizard both lets some magical-luddite chump approach within 30' of him, and chooses to let the momentary-disjunction effect him (out of... sportsmanship? I guess?). Even then he can still cast invoke magic and still use which ever spell he wants.

EDIT: Or the wizard simply casts celerity and destroys the Witch slayer before his little disjunction goes off.

EDIT 2: Or the Wizard has a more direct contingency or contingent spell set up to pull him out of range of the disjunction.

[Slipperychicken]

1. Go first, defeat the beatstick before it acts.

2. Use craft contingent spells.

3. Pull out a scroll of dimension door and get out of the way for a round. Then come back and Wizard the witch slayer to death (in 4 rounds or less, of course).

4. Otherwise flee or make yourself nigh-unreachable for a round, like using gemjump or a phantom steed, then come back and defeat the witch slayer, as per #3.

[Karnith]

What level are we talking here? At the lowest level that a Witch Slayer gets the ability (character level 10), it's not bad, but it scales quite poorly.

Some general tactics/answers:
Making the Will save is entirely feasible, particularly at higher levels. I mean, it targets the Wizard's Will save and the DC is based on Cha. A bog-standard, equal-leveled Wizard's base save is going to be +7 when a Witch Slayer gets access to this ability, and it's not exactly difficult to get it higher.
Staying out of reach works. Momentary Disjunction only extends out to 30 ft, so flying out of range and/or moving really fast (e.g. through a Phantom Steed) is the obvious answer, particularly since that's a good strategy against melee-types to start with. If the would-be mage killer gets too close, the Wizard could (depending on her level) use Abrupt Jaunt (depending on the distance), Celerity, a Contingency effect, or something similar to teleport away even when it's not her turn.
Being undetectable in some fashion (e.g. Invisibility effects) would prevent the Wizard from being targeted.
Having some way of disrupting Supernatural abilities (a Selective AMF, the affiliation bonus for the Knowledge domain, etc.) would prevent the Occult Slayer from using the ability effectively.
There are ways to cast in AMFs (etc.), Invoke Magic being the big Sorc/Wiz one.
Having minions on-hand can buy the Wizard enough time to just outlast Momentary Disjunction.

[Slipperychicken]

Being undetectable in some fashion (e.g. Invisibility effects) would prevent the Wizard from being targeted.

Initiate of Mystra is another option for this, although it requires 3 Cleric levels.

[Kelb_Panthera]

The biggest problems with this idea is the limitations of momentary disjunction.

DC 20+cha just isn't going to be that high on a beat-stick type character and it's going up against a wizard's good will save.

Then there's the 30ft range. That is godawful close. Even close range spells are swinging around 75ft at the minimum level that a witch hunter could use momentary disjunction, forget about medium and long-range.

Then there's the wizardly defenses against being caught by surprise. The most damning of these comes at 17; foresight. Seriously, its a spell that says "No. The wizard is not surprised. Ever." Before that there's greater arcane sight giving them a radar for mystical effects within 120ft unless you have a way to shield the auras of any items you're wearing and/or do the same for any ongoing spell effects on your person.

Many wizards also take a dip into mindbender and pick up mindsight at 6. If you have an intelligence score and are withing 100ft, they know you're there and how smart you are.

If you can get past the defenses and catch the wizard by surprise and if he fails his save, you get one round to drop him. Any magic items he's carrying, such as a staff *hint*, still work just fine and that presents a danger, particularly if he has any crafted contingent spells.

[Rubik]

Anyone who can afford one should buy an Iron Heart vest (from ToB) or have the effect added to a magical shirt (such as a shirt of wraith stalking). Take Iron Heart Surge as your maneuver, using Heroics to qualify via Martial Study. Any way to nab an extra standard action, whether from Celerity, Synchronicity, factotum 8, or a belt of battle, will allow you to kill the effect PDQ, without too much effort.

A couple of rings of Diamond Mind for save-replacing maneuvers would be an excellent way to make the save, especially since you explicitly don't auto-fail on a nat-1.

Also, having hired, summoned, created, or called minions to pull your fat out of the frier is a fairly easy way to keep you safe while you're slightly more vulnerable than normal.

Note that the ability doesn't do anything at all to your magic items or already active magical effects, so you're free to use wands, scrolls, pre-existing buffs, wondrous items, and so on to your heart's content.