You can easily combine a visit to Rosendal and the Barony with a cruise on the Hardangerfjord!
See fjord cruise package that gives you great discounts on the boat ride Bergen - Rosendal - Bergen, entrance with tour of the Barony and a 2 course hot lunch in Rosendal Greenhouse here.
The boat also passes by Flesland. So it may be possible to combine with flights to Bergen Airport Flesland. In the attached PDF you will find the schedule for the boat and the bus.
It is possible to just take the cruise without visiting the Barony. There is bus service between Rosendal and Odda on all departures! And the boat is always waiting for the bus
Between Odda and Rosendal it is bus route 760 with company Skyss. See travel-planner. Select "Odda bus Station (Odda)" and "Rosendal kai (Kvinnherad)".
If you wish to travel between Stana Gard and Bergen without your own car, this is my best tip! (Bus-stop in Stana is named "Stana (Odda)".