。
She partly agrees with this decision.
Literal
She [topic-は] this decision [with-に] partially is-agreeing.
~に同意する ('agree with') takes に for what one agrees with — Japanese consistently routes targets of mental and relational states through に. 部分的に ('partially, in part') is the adverbial form of 部分的 ('partial'), built with the suffix ~的 that turns nouns into adjectives ('-like, -al'). Useful productive form: 部分 ('part') → 部分的 ('partial'). The continuous ~している locks in her ongoing stance — agreement as a current attitude, not a momentary act. A measured, slightly formal sentence appropriate for business meetings or political reporting.