I beat the last boss over 100 times. Apparently I'm not going to be able to beat this one more than 20-30 times. 5 cash hits on L9. I haven't changed tiers, unless L250 is a new tier unto itself.
That is, of course, if I can even get 30 bosses to trigger. It was fine for me at the beginning of the event. Now it's about a boss every 30-40 minutes of energy/stamina constant use.