So, the campaign I'm currently running is a Pathfinder-ization of the 2E module The Gates Of Firestorm Peak with a level 6 party. The party will soon be coming up against a mid-boss type of fight. Unfortunately, the mid-boss was a level 9 dwarf fighter in the original, which is considerably less dangerous relatively speaking in 3E/PF than it was in the old days. I'm looking to work him into a form that will be a somewhat more credible threat.
The fight will probably be a semi-duel: only the NPC and one PC in actual combat (probably the party's barbarian or paladin), but with other PCs allowed to buff as long as none of their spells affect the NPC. The NPC has a special condition which gives him a save-or-die attack, but it takes several rounds after the save is made to kill its target, which means I'd rather not rely on it to give him his threat. I'm willing to grandfather in 3.5 feats and PrCs for this character, but I would want their effects to be relatively non-obvious - that is, if someone was watching the fight and didn't know the behind-the-scenes rules, he could believe that this guy really was just a badass but somewhat insane Fighter (so Barbarian rages would be okay, but not Bear Warrior, for example.) I'd rather avoid any ToB material that's obviously supernatural, as well.
So, any suggestions for feats or PrCs that could make this guy more of a hazard? I don't want him to be a Certain Winner, but I would like him to present a credible threat and be plausible as somebody none of his underlings is willing to challenge directly.