1. Expand inventory slots
2. Spread your low level manyus evenly across all your teams
3. ???
4. Profit

Seriously, save those level 100 manyus at level 50+, in order to reduce the cost of leveling up (exp to cost ratio) When you first start this game, strategically and patiently level up ALL YOUR TEAMS evenly. Don't bone up on just one team and then expect to clear maps that way.

The big key here is to make sure you have ample inventory space. Spend those flower gems to expand inventory more than to roll gachas. You'll get tons of 5★ girls from events (and event repeats) in no time.