Better focus on expanding your harem by completing the story mode and beating the Villains, and do the Champion performances 'coz other than that 1 super Mythic (Bunna) that came out last month : the 3 Girls from the Champions have arguably the best stats in the game (Any, Murane and Shtupra) and they're available for all.
More girls in your harem = more money generated per day = more money to spend on the market to buy Exp Books and Gifts to power-up your harem.
Also : make full use of the Calendar on the Announcements. Strategize and use your resources wisely : tickets, orbs, money. kobans. If you don't have an abundance of these : then check the Calendar which events you'd want to unload your resources on because you cant win them all (unless you pay real cash to win as everyone else is highlighting).
And check out the forum for contests, surveys, etc. for free Kobans.
I've been playing the game for more than 2 years now and I'm a free player. It is doable but it is gonna be really hard. I'm currently level 348 with 231 girls.