I've been looking for ways that a fighter (or other martial class) can fight effectively, preferably at low levels, against other high AC opponents. I know there is things like "Ki Shout", where you can bypass AC and attack via saves, but are there anything else that accomplishes this? Any other special maneuvers or attacks?
Have Your Opponent Cower Before You
Scaring the bejesus out of someone doesn't consider their Armor Class as a means of defense. Getting a way to make them frozen solid from fear would render many armor classes much lower.
And a Zhentarim Fighter is a great reason to actually have 9-10 levels of fighter (arguably the only reason). You get Skill Focus (Intimidate) for free, as well as Extended Intimidation and Swift Demoralization - without giving up anything at all as a fighter.
Example: Intimidation can be very broken. A Half-Orc Paragon Zhentarim Fighter with Intimidating Rage (Complete Warrior), Skill Focus (Intimidate) (Player's Handbook), Imperious Command (Drow of the Underdark), Never Outnumbered Skill Trick (Complete Scoundrel) and Fearsome armor ability (Drow of the Underdark), could end up demoralizing the person with a high armor class into a Frightened, Panicked, or Cowered condition. Instaneous Rage would allow you to do all of that when it isn't even your turn.
A detailed explanation of how to scare people to death is covered by the Fear Handbook.
While the list below isn't 'saving throw' related, it can reduce or render ineffective a high armor class.
If You Can't Hit It, Hit the Ground Beneath It
You can instead target a specific grid intersection. Treat this as a ranged attack against AC 5.
Trip your Opponent
Get Help from a Grappler
You make a melee touch attack to grab the target. You can hold your opponent immobile for 1 round by winning an opposed grapple check (made in place of an attack). Once you have an opponent pinned, you have a few options available to you (see below). While you’re pinned, you take a -4 penalty to your AC against opponents other than the one pinning you.
Tome of Battle includes maneuvers that may help fighting against high AC, especially in the Diamond Mind school:
- the Emerald Razor (2) strike targets Touch AC instead of AC, making it effective against highly armored targets
- the Nightmare Blade strikes: Sapphire Nightmare Blade (1), Ruby Nightmare Blade (4), Diamond Nightmare Blade (8), hits using a Concentration check instead of an Attack Roll; skill checks can be boosted much more easily than AR, up to +30 for 90,000gp pre-epic
The Diamond Mind school is accessible to both Swordsage and Warblade, and of course is accessible via the Martial Study feat, which is a Fighter Feat.
The school also features a serie of counters Moment of Perfect Mind (1), Action Before Thought (2) and Mind over Body (3) which allow to replace your Will save (resp. Reflex save and Fortitude save) by a Concentration check and the Diamond Defense (8) counter which allows you to add your Initiator level to any save, so it is rather well-rounded.