使いこなす。（つかいこなす） [tsu-ka-i-ko-na-su] means "acquire a command." More frequently, mastery is rendered with 「習得」（しゅうとく） [shuu-toku]. "To master a language," for example, would be rendered as 「ことばを習得する。」　（ことばをしゅうとくする） [kotoba-oh shuu-toku su-ru] to master (kanji: learn and habituate a pattern) a language.