It's not impossible. The writer can easily cheat because he is in control of the story. The writer can give characters great deduction skills because the writer already knows the answer to the questions the character has.
All the writer has to do is find a plausible explanation for how the character figured it out, which isn't that hard. A character that consistently shows great deductive skills easily passes for a genius. It's like Sherlock Holmes. You don't have to suspend your disbelief to believe that Sherlock Holmes is a genius.
You have to suspend your disbelief when fucking retarded Johan is just sitting in a chair with his dumb face doing literally nothing as people kill eachother for no reason other than ''hurr Johan is a genius! he a GENIUS!'' That's just retarded.
Fanfic-tier retarded writing, it's like those Sonic fanfics.