Have already killed the troll king (had to use easier mode though), but still haven't found where it is despite most part of the castle, other than blocked by falling rock, had been visited and still nothing at view.

Also the game mentions they turn to rock when sun rises, is it something that we can use such as sunbeam would petrify it, or is it just fluff? Some say slow works on the troll king but even on easy mode it does little to nothing in my play-through, fascinate and fear all fail too.

Many doors also act weird, saying they require key to open. But after opening certain chests that don't have keys in them, suddenly they can be unlocked with skill?

The bow is in the top left part of the map on the first floor, you need a key which is IIRC somewhere on the right side of the map, and IIRC it is hidden (so you need to make a spot check (edit: make that perception)).

Slow and fear both can be negated (at least partially) by a will save, have you looked at the log if he made his saving throws?

Is it behind the 2 small trolls and the big troll place? I got exterminated by the big troll and didn't save. :?

Yeah, that's the place.

Got it now, thank you.

That troll is tough with attack bonus +19 or so, but hideous laughter nails him. :P