。
She'll probably try it again.
Literal
[topic-は] one-more-time that [object-を] do-try probably.
やってみる pairs the casual all-purpose verb やる ('do') with the auxiliary ~てみる ('try doing, do and see'). The auxiliary ~てみる literally derives from 見る ('see, look') — 'do X and see [what happens]' — which is why it's almost always written in kana to mark its grammaticalized status. ~だろう adds speculative confidence ('probably, surely'); the speaker is making a prediction without solid evidence. やる carries a slightly more colloquial, hands-on feel than する; the latter would sound a touch more abstract.