。
She did it entirely on her own.
Literal
[topic-は] completely self-alone that [object-を] did.
自分ひとりで ('on one's own, by oneself') stacks 自分 ('self') with 一人 ('alone, by oneself') for emphasis — 'completely solo.' まったく ('completely, utterly') intensifies the lone-handedness. The whole sentence praises (or notes) self-sufficiency — the kind of remark that might appear in a teacher's evaluation or an admiring observation. Cultural undercurrent: Japanese culture has historically emphasized group effort and collective accomplishment, so 'doing it completely on one's own' carries a subtle weight — it's notable enough to be commented on.