I've found that really only getting the first girl is easily done by a truly free-to-play player without saving up an absurd amount of Kobans ahead of time. Without spending any Kobans (other than to skip the resting between performances against champions). Before I began any of the events I looked through everything and tried planning ahead. It helps that the rewards per stage actually help set you up for the upcoming stage by providing a decent start, but the 40 Champion Performances is difficult no matter how you look at it.
In summation, if you're tryna 100% Boobies Wars as a free player...rip