I have a friend who spent a year studying in America before she came here and she struggles a lot and she's very intelligent, so be aware of that. Experience of exams is the thing that holds her back, she has basically no practise of writing under timed conditions.
I'd get started as normal with a bit of work over the summer, but as soon as you encounter a problem start working on it immediately. My friend didn't realise until the spring that timing was really going to be a problem, and by then she'd missed three available months for improving on it.