The easy way: go for the high CPU instances (don't bother with the 'regular' ones, other providers offer cheaper and better machines)

The slightly harder: go for technologies that help you spread the load, so you spawn several instances that run simultaneously, run for some time and them stop, costing you maybe what would cost you for 1 instance/month but giving you the results faster

The even harder way is going for spot instances and orchestrating the sharding and reassembly of results