The problem with the Fighter class is that it has a very limited toolbox. Sure, you're strong, and can beat up most enemies you face. But when you run against something that uses unconventional tactics... you don't have many options.
There are still a few things you can do, though.
Firstly, you can multiclass.
A character like Darvin suggests is almost as strong as a plain fighter and MUCH more flexible. (Though you might want some more fighter levels than he suggests. His build is pretty good, but more fighter levels is more forgiving.)
If you have rogue levels and the Use Magic device skill, you can use scrolls and wands. FOr example, against that wizard, you can look for scrolls of Resist/Protection From the Elements, to block his fire. You can find scrolls of See Invisibility and Lesser Spell Breach to breach his defences. You might even be able to find scrolls of (lesser) Spell Mantle to simply absorb his first few spells.
Secondly, you can use magic items and potions. This is definitely important for big boss fights. Drink potions of Bull's Strength, Aid and Bless, and suddenly you'll be interrupting his spells a lot more often. Potions of Endurance help you soak up damage, and potions of Clarity stop fear/confusion/paralyse spells from working on you, which is a huge lifesaver. A potion of Invisibility will let you sneak up on the mage and get the first attack in. This makes a big difference against spellcasters.
Moral of the story: Buy a magic pouch and stuff it with 10 of every kind of potion once you can afford to.
Other items also help. Use a two-handed weapon even if you don't normally use one, for you don't need a shield against spellcasters and two-handers can bash through his protective spells such as stoneskin much more easily.
There are also many magic items for sale in shops that can be a great help against specific foes. For example, belts of guiding light can stop instant death attacks, and amulets of health can stop poison. If you find a Bone Ring, hang on to it at all cost.
Think outside the box if you have to. The wizard store sells Robes of Fire Resistance. And you don't need to be a wizard to wear them. And since wizards don't hit you with physical attacks, who cares that your armour class goes down? You may not win the Fighter Fashion Award, but at least you won't burn to death.
Thirdly, you should fight smart. Just because fighters are as subtle as a half-brick in a sock doesn't mean you should just charge in and hope for the best. That way lies the respawn screen.
Boss has a bunch of henchmen? Run away and try to lose some. Then turn around and fight the boss one on one.
Archers ruining your day? Take cover behind a rock and watch the fools run right into sword-range.
Wizard just cast Haste? Run away and wait for the spell to wear off. Then come back and demonstrate to the wizard why it sucks that he can only cast his spells once a day. By hitting him in the face with a greatsword.
Also, get the Knockdown feat. That one ruins every wizard's day.
Fourthly, you can come back later with a bigger sword. That one almost always works in games like this. Some boss fights are just very, very tough. So go somewhere else, get another level or two, gather some gold and buy more items, and then come back when the odds are in your favour. Sporting? maybe not. Effective? Indubitably.
Anyway, I hope that helps. The game is quite fun, and you have more options than you think.
Now for some specific-but-less-important stuff.
Makatak: There are alot of races/classes/skills/feats to choose from so I just clicked the 'recommended' button for everything to play as a Chaotic good human fighter with standard kit. I do the same thing whenever I level up. I'm using the maps at gamebanshee.com as a guide so I don't miss side quests. And I've hit a roadblock at the prison. The spellcasting prisoner I had to attempt over and over until I got lucky and interrupted his casting 2 or 3 times in a row with myself and my henchmen Sharwyn. Then I got to the Orc and had the same problem. Is this a bad build? Or am I missing something?
This is NWN diamond I'm talking about.
Engerek01: Recommend button works fine, dont believe others. I know the caster you are talking about. He is in the middle of the second level near chests and he throws fireballs. I can easily say that he will be the hardest opponent you will meet in the entire game and the only NPC that I keep dying to myself. (except monsters that instant kills you but there is nothing you can do about that).
EDIT: Only Downside of recommend is it puts points into Parry which is useless.
The recommended button works... barely adequately for SOME classes, and not at all if you want to multiclass. Most of the time it will give you a very mediocre, but workable character if you stick to a single class. But god help you if you want to play a sorcerer and rely on that button for your spell selection...
(But it's usually pretty good for assigning stats during character creation, so that's something.)
As for instant-kills: there's almost always something you can do. Whether it's buying a magic item, or learning tricks to deal with it.
For example, a Morgh has an instant death-gaze. Clerics can cast Death Ward, but what if you're not a cleric? Well, you can always run away and shoot the damn things. Or lure them with an arrow, then jump them when they come around a corner, using knockdown. Or use a potion of invisibility to sneak up and attack. With knockdown. But even if knockdown doesn't work, once you're in melee range the Morgh will probably try to beat you instead of using his instant-death gaze.
Water Elementals are even easier. They have an area effect instant death attack that is very annoying, but huts them and is one use only. So if you angle it right, you can run PAST them, make them trigger their instant death attack whilst you're out of range. Then they're wounded and have no more instant death on the menu.
The only instant-death attack I can think of that has NO counter whatsoever is the Balor's (or Cleric's) Implosion spell. That one just sucks. Better hope your fortitude save is up in the stratosphere. Or use a scroll of spell-mantle.