Your hyper is one of the most powerful hypers in the game. Early on it is the only way to get beyond your lackluster stats with its attack boost. Your early game should focus on drawing cards first and foremost.
Battles should generally be avoided, especially if not holding a hyper due to native -1 ATK.
Cards such as Tactical Retreat and Rbits are incredibly valuable at blocking off potential early game assaults, while your above average evasion helps against lower rolls and most early encounters. While they can also counter Sweet Eater's onslaught in the late game, they will also additionally help her not lose as much HP, which is crucial when trying to maintain the transformation, as neutral DEF and EVD is not as spectacular for surviving constant assaults as one might think.
Playing As Tomomo (Sweet Eater)
After level 3, your hyper becomes an insane transformation, along with a full heal, 4 additional attack, and a new hyper card that acts as a less predictable air raid based on health you're missing. Depending on where casual mode left you, Sweet Eater can either hunt people down for wins or farm stars with an imposing statline to both hound loaded opponents and discourage attacks from others.
Despite these massive changes, you are still left decently vulnerable due to the punishing circumstances if you do die, as well as a lack of defensive boosts. Because of this, defensive cards like Rbits can help tackle other offensive characters like Star Breaker or Yuki.
Any card to get you into more battles or increase your chances of scoring a KO during one is a huge asset while controlling Sweet Eater. This includes mostly any offensive battle card, though watch out for your opponents gaining these cards as well, as Casual usually struggles to survive the likes of attacks boosted by I'm on Fire!, Accel Hyper, or Big Magnum.
Playing Against Tomomo (Casual)
Hound her with battles nonstop, as her stats in this form are lackluster.
Out of battle KOs return Tomomo (Sweet Eater) to Tomomo (Casual). This includes cards such as Gamble! and Long-Distance Shot. Ironically, she is especially prone to her vanilla self's Magical Massacre, as the transformation to Sweet Eater also resets her to full HP.
Neutral DEF and EVA leaves her somewhat vulnerable to opportunistic attacks from bullies and loaded attack rolls via cards such as Big Magnum and Accel Hyper, especially if she's lost some HP beforehand.
KOing her empties her hand, reverts her to Casual form, and likely sidelines her for several turns due to 6 REC as Sweet Eater.