So, is the sentence idiomatic? Can we use it for anyone who is strong?
When I tried to understand the sentence myself I concluded that the speaker disliked her so much (for whatever reason) that he doesn't even want to meet her in the dark, where he, of course, cannot even recognise her (properly).
It means the person is big and strong and can beat you up. The reference to the alley is that there is no one to see the person beat you up so he (in this case, she) can do it without attracting attention.