The best way is going to be difficult regardless of how you do it. Which Berlin airport?
I would suggest the quickest way would be via Berlin Hauptbahnhof-Angermunde-Szczecin-Koszalin, but this is going to take well over 6 hours for train.
I've no idea about buses, but there are usually direct buses from the Berlin airports to Szczecin, from where you can get a train - but as I say, you'll be about 3:30-4:30 from Szczecin to Koszalin by train.