Optimised useful warrior

[Glodart]

I want to know, is it actually possible to make a Fighter, Barbarian, Ranger or Paladin useful out of combat and being able to render the casters useless by killing them before they cast.
Any ideas would be great, because I intend to make some sort of Mage Slayer eventually

[Seerow]

I want to know, is it actually possible to make a Fighter, Barbarian, Ranger or Paladin useful out of combat and being able to render the casters useless by killing them before they cast.
Any ideas would be great, because I intend to make some sort of Mage Slayer eventually

The first half is possible. Second half is not against an optimized caster.

[Flickerdart]

The problem with "killing a spellcaster before they cast" is that the vast majority of spells have durations, many of them lasting hours or days. You are taking the correct approach (it is a much better idea to stop the enemy from using magic rather than try and deal with the consequences of spells) but it's still a very difficult thing to do, especially if you are not willing to be a full caster yourself. You might be able to get around that by employing a large number of people to arrange circumstances into an elaborate trap that's difficult to see without divination abuse (a staple of any high-op wizard) but this is no easy task without falling into the trap of "I hire a wizard to...".

I recommend reframing the problem. Not all enemies you face will be spellcasters proper, but many might share some or all of their abilities. You might have a decent amount of luck fighting pre-made monsters that follow similar tactics to what a real caster might use.

The first step is detecting the target, ideally before they detect you. Mindsight is the go-to sensory method for many custom monsters, but blindsight and similar are more popular among stock MM types. Monsters also tend to have high HD and therefore a lot of skill points to put into Spot and Listen. They will also have home field advantage in terms of environmental conditions - good luck finding a dragon in an unlit cave! Acquire an ability to detect creatures reliably beyond 100 feet and without needing a line of sight, and you're ready to move on to the next step.

Once you've found them, you want to close in. Archery can work in contrived circumstances if cheesed out (there are a couple of OHKO builds out there) but then the rest of your party doesn't get to do anything and it's kind of a bummer. Charging is a stupid idea for many reasons. The Horizon Tripper build accomplishes this with a Dimension Door SLA. For best results, bring the party within operational range when you do this.

Hopefully, the monster you cornered can't cast Contingency, because there's nothing you can do about it. Likewise, Foresight means you're screwed. Generally, try to keep this to the lower levels where these things aren't as common.

If you still have an encounter after the Contingency went off, your first order of business is to prevent them from casting. Simply threatening them is useless - Concentration checks to cast defensively are really easy, and properly buffed casters (remember, buffs last a long time - the caster may have applied them in the morning or even the day before, and with Extend Spell or a high CL, they're still around) will scoff at AoOs you make. Silence is a good tactic - while many casters will have an answer to this (especially monsters relying on SLAs), many others will not, and it doesn't cost that much.

If they can't cast, they'll try to run. Casters are very good at being mobile, with various magic that lets them fly, burrow, or teleport. At the very least, make sure they can't tumble or 5ft step away. Tripping helps. Immediate-action teleportation helps. If you can dazelock them, so much the better.

It's very likely that after all the build resources needed to pull this off you're not actually that handy in a fight. But now that you've got them nailed down, a buddy can finish them off.

Notice how I never mention antimagic fields. That's because they're super dumb - a 10ft radius field emanating from you (which has to be cast by you) that turns off all your magic items and abilities hurts you more than the guy you're going after.