You can beat every map imo with even 4* Stars. But to optimize Gameplay it's a wise decision to make a synergistic squad.
The difficult part of this game in my opinion is to get a strong Team which also takes waifuism into account.
And btw. you don't even need Rainbows to create a synergistic Sqaud.
For example:
Moonvine, Tachibana, Geranium are good in a Team based on Skill Activation Rate abuse and all three are Event Girls
You could post your current Squad here and we can help you to optimize it:
http://harem-battle.club/flower-knig...m-tactics.html