Intimidate/Demoralize Opponent is worse than useless in one-on-one combat because it uses up a standard action and it "wears off" before the intimidator gets to act.
But if you have two or more intimidators, it can be terrific.
For example, three fighters with spiked chains are threatening a wizard. (If he's not scared already, he should be, but that's another story.)
On fighter 1's initiative, he does a Intimidate/Demoralize Opponent check. The wizard gets to oppose with modified level check -- (1d20 + character level or Hit Dice + target’s Wisdom bonus [if any] + target’s modifiers on saves against fear). If fighter 1 wins, the wizard becomes shaken for 1 round.
A shaken character takes a –2 penalty on attack rolls, ability checks, and saving throws. This normally isn't a big deal for a wizard in a wizard vs. fighter match because the fighter isn't normally gonna force the wizard into attack rolls, ability checks or saving throws.
Now, on fighter 2's initiative, he does the same thing. The wizard gets to oppose with modified level check (probably at -2 this time because he's shaken), but this time, if fighter 2 wins, the wizard is shaken again.
Fear effects are cumulative. A shaken character who is made shaken again becomes frightened, and a shaken character who is made frightened becomes panicked instead. A frightened character who is made shaken or frightened becomes panicked instead.
So since the wizard is already shaken by the first Intimidate/Demoralize Opponent, when fighter 2 gets him, he goes to "frightened."
Characters who are frightened are shaken, and in addition they flee from the source of their fear as quickly as they can. They can choose the path of their flight. Other than that stipulation, once they are out of sight (or hearing) of the source of their fear, they can act as they want. However, if the duration of their fear continues, characters can be forced to flee once more if the source of their fear presents itself again. Characters unable to flee can fight (though they are still shaken).
The wizard can't flee until his turn in the initiative order.
Now, if fighter 3 tries the same thing, the wizard makes another opposed check (probably at -2). This time, if fighter 3 wins, the wizard moves from "frightened" to "panicked."
Characters who are panicked are shaken, and they run away from the source of their fear as quickly as they can. Other than running away from the source, their path is random. They flee from all other dangers that confront them rather than facing those dangers. Panicked characters cower if they are prevented from fleeing.
So as the wizard runs away, he's likely to provoke attacks of opportunity from all three fighters. (Remember, they must threaten the wizard in melee combat to do this anyway.)
This tactic is especially effective with Enlarge Person since large creatures threaten a larger area. You gain a +4 bonus on your Intimidate check for every size category that you are larger than your target.
A seventh-level fighter with max ranks in Intimidate (+10) and five cross-class ranks in Bluff (+2 synergy for Intimidate) has Intimidate +12 (assume no Cha bonus). That makes for about a 50% chance to demoralize a wizard of similar level. With Enlarge Person, that goes up to a 70% chance.
With two fighters trying this, the probability of at least one succeeding is 1.0-(0.3*0.3) = 0.91. The probability of both succeeding is 0.7*0.7 = 0.49.
With three fighters trying this, the probability of at least one succeeding is 1.0-(0.3*0.3*.03) = 0.973. The probability of at least two succeeding is 0.91*0.7 = 0.637. The probability of all three succeeding is 0.7*0.7*0.7 = 0.343.
(My probability is a little rusty, so I might have the formulas wrong, but these percentages seem about right.)
So that's about a 64% chance that the wizard will be frightened and a 34% chance that he'll be panicked. Either way, he's gonna spend some time running away from the fighters.
If the wizard has Stoneskin, Mirror Image and Fireshield up, the fighters might have a hard time beating him down in melee combat, but if they can make him run away for a bit, they can hide until his short-term spells exire or use ranged weapons or whatever.
Using Intimidate/Demoralize Opponent can also be very useful in conjunction with a Fear or Cause Fear spell, or similar magical effect which causes fear. Many of these effects cause the target to be panicked unless it succeeds on a Will save, but if the Will save succeeds, the creature is shaken for 1 round. With that, a successful Demoralize Opponent can make him run away.
Player characters have more defenses against fear than NPCs and monsters, making these useful tactics to know. Defenses include paladin goodness, Cloak of Bravery and Heroes' Feast spells.
