SOA analysis for MOSFET-based Electronic Load
The long term heat equation uses the Rjc+Rca (jcn to case and case to ambient to achieve the temperature rise you want. 

Although it is rated for Tj =175'C abs. max in the SOA curves, you likely want run the case temps around 85'C max or a rise of 60'C and a heat sink of &lt;2 deg C/W. 

This heatsink will be large or small with a fan like those used for CPU's.