。
She did it with ease.
Literal
She [topic-は] that [object-を] [effortlessly-楽々と] did.
楽々 (rakuraku) is a reduplicated mimetic-like adverb — 楽 ('comfortable, easy, pleasant') doubled — meaning 'effortlessly, with ease.' The と after it is the adverbial と (often optional), tying it to the verb. Common collocations: 楽々と歩く ('walk with ease'), 楽々とこなす ('handle with ease'), 楽々合格 ('pass easily'). The word feels mildly admiring, as if the speaker is impressed by how light the task seemed. Reduplication of single-kanji adjectives is a productive pattern in Japanese: 早々, 続々, 着々, 度々, all giving emphatic adverbial forms.