Come across – meet someone accidentally.
Ex: I came across Martha at the meeting yesterday.
Come down with – start being ill.
Ex: I’m afraid I can’t go out with you tonight. I came down with the flu.
Come forward – to offer information about something.
Ex: No one came forward to witness last night’s crime.
Come up against – to face a problem you have to deal with.
Ex: If you come up against any difficulties, give me a call.
Come up with – to suggest an idea.
Ex: Brian came up with a great idea of how to double our income.