。
She took aim at the target.
Literal
She [topic-は] target [at-に] aim [object-を] fixed.
ねらいを定める ('to fix one's aim / take aim') is a set phrase combining ねらい ('aim') with 定める ('to fix/determine'). 的 ('target') takes the に particle as the thing being aimed at. The phrase works for both literal contexts (archery, shooting) and figurative ones (setting one's sights on a goal).