。
She's very good at the violin.
Literal
She [topic-は] violin [subject-が] very skilled is.
A textbook example of the は + が construction with stative predicates of ability and preference: 上手, 下手, 好き, 嫌い, 得意, 苦手 all take their object with が rather than を, because they are adjectives expressing a state rather than verbs taking a direct object. The topic 彼女は sets the frame ('about her'), and the が-marked バイオリン names what the skill applies to. 大変 ('very, greatly') here is an adverb intensifier; it can also stand on its own as a na-adjective meaning 'awful, terrible.'