What's the best way to tenderize meat(challenging question, read on))?

I'm charged with the task of making fajitas tonight. It's Easter Monday where I am so there are absolutely no shops or butchers open. The beef that they(my friends) bought is London Broil. I fried up a test piece(a strip) and it was as chewy as rubber. Is there anyway to tenderize it before 8:00 tonight?