Magit does interactive staging (and unstaging) a lot better than git itself does.

In d u, you can “s” on a file or hunk and it’ll stage just that. And if you select lines (c-spc?) it’ll stage just those lines.

To unstage, go to d s and use “u” the same way.

The massive advantage aside from line-wise staging is that you don’t need to stage linearly.