I don’t think your cruise is starting from portofino. It’s a tiny, VERY expensive harbor for mega yachts but too small for large cruise ships. Most likely it is staring from genova, please recheck.
If it is starting from Genoa, where MSC cruises usually start, then the train station you want is Genova Piazza Principe, which is the station before Genova Brignole. So get off the train one stop earlier.
From Piazza Principe, the cruise terminal is ten minutes walk without luggage, but with luggage a taxi is advisable.