The phrase "let-alone" is typically used as a conjunction to emphasize that something is unlikely to happen, or that it is not worth considering alongside something else. Examples might include phrases like "I couldn't afford a car, let alone a house", or "I don't have time to read for fun, let alone watch TV". Synonyms for "let-alone" might include phrases like "much less", "let alone" and "not to mention". Other options might include phrases like "far from", "apart from", "excluding" and "aside from". Depending on the context of the sentence, there may be other synonyms that could be more appropriate, such as "not even", "never mind" or "not to speak of".