So my players recently fought a big bad campaign boss. The (custom made) boss was a kind of solo beserker king, and the encounter difficulty was designed to be fairly moderate until the boss became bloodied, at which point he would frenzy and dramatically increase his damage output and the encounter difficulty.
However, once the boss was bloodied, the first thing the bard did on his next turn, was to use Knack for Success on a intimidate check. With a high roll, a naturally high charisma score and a +4 from KfS, he easily beat the simpleton/barbarian boss' Will (even with the +10 bonus for being hostile).
I hadn't considered this at all. This was the first time in maybe 8 levels that any of the players had used their Intimidate skill. The player in question was pretty excited as he had obviously studied and prepared to do this. He asked me directly, out of character, if the boss was immune to fear, which it wasn't (it seems some solo creatures are and others aren't – the solo monster I had used as a template wasn't).
I though it would be bad manners to suddenly add an immunity to the boss, especially after the bard player had prepared this strategy, but it really bummed out a few of the other players (and myself) who had saved resources and looked forward to the epic campaign-ending encounter, only to see it end prematurely and in such an anticlimax.
In the future I will definitely throw immunity to fear in with all important bosses, but I am wondering if there was anything I could have said or done in this specific situation that would've been better.
Another clarification: I did have the bard roleplay his intimidation speech, and it was pretty good. Not worthy of bonuses or penalties, but solid. The player was completely following RAW.