If all you care about is gold, then the gold mission (ideally while running gold boost as well) is definitely the way to go; during the boost time I usually end up earning about 1.5M gold on average (burning about 5-6 FGs as well) with the slowest squad clocking in at 561 speed.

Otherwise, I second Volarmis' suggestion about running 80 stam event stages (particularly in phase 2).