i have only beat bosses 8 but that cost me about 9-12 dolls, and i only do is it about once every few weeks when i have the spear dolls and want the inventory space back,
have not even thought about taking on the 9th on yet will not do that till i meet the following
1. closer to the same lvl
2. can beat the 8th with only a 1-3 dolls used