usageaster \YOO-sij-as-ter, noun:

A self-styled authority on language usage.

Newman went on to voice sentiments held by other usageasters: I think that slang adds richness and originality to English.
— Charlton Grant Laird and Phillip C. Boardman, The Legacy of Language

A poetaster pretends to write poetry; a usageaster pretends to know about questions of usage in language.
— Allan A. Metcalf, Predicting New Words

Usageaster is derived from the word usage and the suffix -aster which refers to something that imperfectly resembles or mimics the true thing.