Example John using intmidate on Gregor. Rolled his skill and got 50. Big number. However... Gregor just laughing. Why? I'll explain.
DC of intimidation = Target level+His Saves against fear (Will saves+bonus checks against fear) +1D20.
Math aside - to intimidate an average 30 lvl character you need to beat 60+dc. If you cant - i think its better to not even try. You are not that scary for seasoned adventurer
Q. I used intimidate without any problems at NWN2 original stories. Why its hard to scare another player?
A. Social skill checks never meant to be used against other players. And players usally more powerfull than bunch of thugs and poor baker you scared
Q. But i scared of a bunch of ogres. They looks strong! What the problem?
A. Exactly. Challenge raiting of ecnounter is based on power of enemies and their quanity. So at level 20 and maxed intimidation you scared a ogre leader, which had 10 lvl and loads of about same level troops.
P\S Fear Immune Creatures\Classes cant be intimidated at all.
P\P\S Its possible to intimidate other players of 30lvl. But you need to put a lot at indimidate skill. Get a gear for it. And yes, feats.