Ask HN: What's the best way for a foreigner to improve English to a great level?

My wife and I are Italian, and we live in San Francisco. She's trying to find a job but feels stuck because her English is not good enough (I think she's at an acceptable level, but clearly she can't speak or listen as good as a mother tongue). She's looking for advice on what's the best way to dramatically improve it in 4 months, 25 hours a week or so. Thanks!