Apr 16, 2019, 14:2604/16/19
From my understanding.
It is just completely random no matter the stage.
Although i am not sure if Difficulty changes things.
I do know that your better off farming anywhere in stages 1-6 then farming the boss on Stage 7.
I would either farm stages from which you need gear or look at Experience Output and Gear Silver Value for each stage then farm the one that provides the most benefit for raising Food Heros.