Proposed tactic: When you feel that your melee fighter is anywhere near an improved-invisibility arcane artillery-peice (Sorcerer, Wizard, it doesn’t really matter), try the following: Instead of taking your normal standard action or full-round attack, take a movement action to near where you think the Sorcerer is. Use your standard action to prepare a “partial charge” upon the spellcaster’s attack.

Once the spellcaster attacks, your fighter knows which 5′x5′ area the spell came from, and can pursue. Move to the spellcaster and grapple, don’t just swing with your sword. If you land a grapple, the Sorcerer will be limited to non-somatic spellcasting, and he is not likely to fare well in a grappling contest (your strength and base attack bonus are both likely to be much better than his). Proceed to break his scrawny little neck, or stab him repeatedly with a backup dagger or short sword.

This tactic is likely to work better than most other conventional Fighter tactics because the sorcerer isn’t going to want to get himself caught in his own fireball, so he is likely to use his move action BEFORE casting (in the cast of a maximized fireball, that’s a full-round action anyway and he’s likely to take his 5′ adjustment first to get some clearance away from his own blast (the closer you get to him to start, the better.

You’ll have a 25% chance of missing the Sorcerer even if your attack is good, but if you hit, the Sorcerer has negligible chances of getting away. Unless your DM is a total jerk, he should reward you for this kind of tactical thinking. If he’s just out to kill your Fighter, no matter what you do you’re going to get dead, though. This is just a way to improve your chances of success.

By introducing a couple of “utility” feats like Blind Fighting and Improved Grapple, mid-level Fighters can fare much better against similarly-leveled spellcasters. I hope that your 13th level Fighter didn’t pursue a rigid weapon-mastery/Power Attack feat progression if your DM likes to throw Improved Invisibility opponents at him.