。
She kicked him as hard as she could.
Literal
She [topic-は] him [object-を] with-all-might kicked-flying.
思い切り (おもいきり), literally 'cutting-loose-thoughts' (as if abandoning all restraint), is an adverb meaning 'with all one's might, to the fullest, as hard as one can.' 蹴飛ばす (けとばす) is a 複合動詞 (compound verb) combining 蹴る ('kick') + 飛ばす ('send flying') — the second verb adds emphasis on the force and trajectory. Together the two paint a violent, full-strength kick — much stronger than plain 蹴る.