My idea for Sleepy-chan:

I don't have Sleepy-chan.
The only Weakness Damage Girls i have atm are Candytuft and Pumpkin Girl so take everything i say with a grain of salt.

In my opinion she works really good in almost every team due to the fact that she buffs the damage potential of every team except solarblast.

You could also create a Weakness Damage Team like this:

http://flowerknight.wikia.com/wiki/Sensitive_Plant
http://flowerknight.wikia.com/wiki/Pygmy_Water_Lily
http://flowerknight.wikia.com/wiki/Burning_Bush or http://flowerknight.wikia.com/wiki/Evergreen_Candytuft
http://flowerknight.wikia.com/wiki/Curcuma
http://flowerknight.wikia.com/wiki/D...er_(Christmas)

This team has a really good chance to give all your team mates all the Weakness Damage Buffs (RNG is still involved but your chances to get all Weakness Damage Buffs on every Team member is much higher due to the fact that you buff every Damage Type at least twice in this team).
This Team also is a First Turn Squad which means it's not suited for long drawn out battles. Dusty Miller doesn't Buff Weakness Damage but she is a n1 finisher.

There are a lot more ways to build a Weakness Damage Team. It's just my take on it