MOSFET - calculating heat produced by MOSFET
If you use Rth(j-c) you either have to know the maximum case temperature or you have to add the thermal resistance of an external heat sink (case to heat sink + heat sink to ambient) and the maximum Ta.
So, you would use  Rth(j-a). If you are getting too high a Tj using  Rth(j-a) at the maximum Ta then you require a different package or a heat sink.
Don’t forget to account for switching losses and the large increase in Rds(on) at high temperatures and ensure you have sufficient gate drive voltage to get guaranteed Rds(on).