There is a solution unfortunately the PC can't make their skill check. Me I'd expect them to use fire and acid right off the bat as I planned it, or rolled it on a random encounter chart. Or would you have put an out in there in case they couldn't roll a skill check? Its possible they are not using any fire and don't have an option to put 1+1 together.How is that good DMing? Setting a challenge that there isn’t a solution to? Trolls are just a metaphor for any challenge where knowledge wins.
I'm just curious how far the separation goes. No wrong answers, it comes down to style.
On a similar note, and I get this from time to time. A player says "well I didn't bother to make a note of this or anything but my PC should remember what the clue to this puzzle is..." or things to that effect. I don't remember but my PC should type thing. Do you fill them in?