WordNet records no synonym and no antonym for mastering. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
becoming proficient in the use of something; having mastery of
“his mastering the art of cooking took a long time”
the act of making a master recording from which copies can be made
“he received a bill for mastering the concert and making 100 copies”
Synonyms, antonyms and the broader and narrower relations for mastering come from Princeton University’s WordNet. Full sources and licences.