score:6
During the strike there is restricted timetable - so you maybe have to wait longer, but you can get to Mainz by train. Take S8 or RB/RE to Mainz Main Station (Mainz Hauptbahnhof). According to Deutsche Bahn S8 will run every hour.
Another option would be to get a ride. Check blablacar.de standard link from the city or better this one, from the airport there are few rides from Frankfurt Airport to Mainz.