My best character right now is Cerebella, and I've done quite a bit research about her. What I've found so far is that she mainly performs short, powerful combinations that usually begin by sending her opponent into the air and absolutely wrecking them. These combos will usually include some short fast hits, grapples, and (usually at the end) a blockbuster.
I've recently found a pretty good combo that does a little over 8k damage, but it takes a lot of getting used to. The combo is
j.HK > cr.LK > cr.MP > cr.HP > j.MK > j.HK > cr.LK > cr.MP > cr.HP > j.LP (continuous) > j.HP > cr.LK > cr.MP > cr.HP > DP LP +LK > QCF PP
1. The first j.HK is a jump-in (see basic information)
2. The j.LP (continuous) should be done until your opponent lands on the ground
3. The final move, the QCF PP, should only be done if your opponent is stuck in a corner; if this is not the case it's a waste of your special.
Here's a more in-depth vid:
Rena's gonna take you home!