。
She's a genius when it comes to math.
Literal
She [topic-は] mathematics [when it comes to-にかけては] genius is.
~にかけては means 'when it comes to' or 'in the area of' — it marks a specific domain in which someone excels. The は adds emphasis: 'in math specifically, she's a genius (whatever else may be true).' This is a strong, focused compliment about a particular ability.