"Simple" thermal analysis of component on PCB over time
20 Watt-seconds/10seconds * (Rjc+Rcs+Rsa) = 2[W] * Rth(total)['C/W]  = temp rise

I^2*RdsOn is one of your choices in addition to choice of mechanical thermal resistances to apply the thermal version Ohm's Law for convection based on datasheets and after you define the max Tj rise for reliability concerns.