Wizards generally have two patterns of hunting, the active and the passive role. When active, the Wizard will try to kill enemies with his strongest spells, whereas in the passive role he'll be trying to divert enemies and occasionally cast spells such as Fas Nadur. In the active role, the main goal of positioning tactics would be to conceal oneself so one doesn't have to move too much. When in the passive role, your goal would be very similar to that of a Priest - to cut off enemies before they can reach the Warriors/Monks of your group.
Below are three examples of both passive and active roles as a Wizard.

Example 1

A typical example when hunting alone. You (blue) meet up with a lone enemy (red) near a tree (grey) in the Woodlands. Since you're hunting alone, obviously you'll be in your active role, thus your objective is to conceal yourself.

I believe it's quite evident that this would be a very unsuccessful way of accomplishing that. In this example, the Wizard thought of casting his spells before he thought of taking cover - a very bad idea, I might add.

Had the Wizard instead taken cover as shown above, the enemy would be locked in place and he could easily take it out at his own pace.

Example 2

Basically the same situation as example 1, but this time in the Mileth Crypt and with two enemies against you. Again, you'll be in your active role.

As always, focusing on your casting and not on taking cover can get you in some pretty bad situations. Running and casting on one enemy might be a simple job, but with two enemies you can get in trouble pretty fast.

However, take a stroll along the wall and position yourself like so and you won't have to run at all. The enemies will be walking along the wall, as denoted by the bright red fields, trying to get to you, but to no avail. This way you can kill them at your own leisure.

Example 3

Finally an example showing you in your passive role. This group, consisting of you (blue) and two Warriors/Monks (green), will probably focus on their ability to kill and your ability to cast Fas Nadur, thus making any offensive spells from you unnecessary.

However, contrary to what most Wizards seem to believe, this does not mean your only job is to cast Fas Nadur. Most Wizards simply do as illustrated above: stand by while the Warriors/Monks are assaulted from all sides.

Instead, position yourself as illustrated above. It doesn't require anything of you really, except you have to stay there and you can't cast any offensive spells (otherwise the enemies might target you). This way the enemy to the right will be blocked off, letting the Warriors/Monks concentrate on one at a time.