。
She mastered typing.
Literal
She [topic-は] typing [in-に] mastered.
熟達する ('to become proficient, attain mastery') is a Sino-Japanese verb of acquisition, distinct from 上達する ('improve at') and the lighter うまくなる ('get better'). The に marks the domain in which mastery is attained: タイプに熟達する ('master typing'), 英語に熟達する ('master English'), 武術に熟達する ('master martial arts'). Used in slightly formal evaluative contexts — a CV, a letter of recommendation, a respectful biographical note. タイプ is a clipped loanword meaning 'typing' or 'typewriting' (different from タイプ as 'type/category'); context disambiguates.