Stumper
Part of speech: noun
Definition: One who stumps, or speaks, or orates, as a politician. A difficult puzzle or problem.
Poser
Part of speech: noun
Definition: A particularly difficult question or puzzle.Someone who, or something which, poses; a person who sets their body in a fixed position, such as for photography or painting.= poseur; someone who affects some behaviour, style, attitude or other condition, often to impress or influence others