。
She mastered the knack of dealing with difficult customers.
Literal
She [topic-は] difficult customer [object-を] handle knack [object-を] mastered.
An interesting hybrid expression: コツ ('knack, trick,' colloquial native vocabulary) paired with マスターする (verbalized loanword from English 'master'). Such mixing is common in modern Japanese — neither the colloquial nor the loanword is alien to the other. The verb 扱う ('handle, deal with') applies to people, situations, and items.