。
She got out of breath from going up and down the hill.
Literal
She [topic-は] hill [path-を] up-down-doing [and-て] breath-cut [subject-が] did.
登り下り is a compound noun coined from two verb stems back to back: 登る ('go up') + 下りる ('go down'). The pattern is productive — 行き帰り ('coming and going'), 出入り ('going in and out'), 上り下り ('up and down'). Adding する converts the compound back into a verb. 息切れ ('shortness of breath') is itself a compound noun (息 'breath' + 切れ 'cut/run-out', the masu-stem of 切れる); it pairs with がする to mean 'to get out of breath,' parallel to 頭痛がする ('have a headache'). The を on 丘 marks the path of motion — the same を used with 道を歩く.