Foci will make you look really pretentious, but that's just my opinion. Markers might like pretentiousness.
I would go with focuses. The 'uses' plural is correct in English. The latin plural is more complicated than simply 'us' becomes 'i', so if you go with focuses you're guaranteed to have the correct plural, but if you try for foci there's a chance you'll be wrong. Plus, it's possible that the marker will be confused about what you mean, which would impact impression marking and disrupt the flow of your essay.