OMORI

Sweetheart is one of the hardest bosses in the game, because of her mace swing attacks, making everyone Angry skill and having a lot of HP, but she can be beaten easily with a strategy of having a completely Sad party, since she becomes permanently happy later in the fight. To beat her, you need your party to be at least Level 15 (but I highly recommend to be higher level, I beat her when I was Level 20), have Hero use a Frying Pan (he can get this during the cooking sidequest in the right side of Sweetheart's Castle), because it gives him more hearts and makes snacks more effective, have Rain Clouds or Poetry Books (you can buy this from Smuggler Jash) to make your party Sad (because Sweetheart can become permanently happy during the fight, DO NOT let anyone be Angry, use a Poetry Book or Omori's Sad Poem), and have several snacks to restore hearts and juice (you can buy Mango Smoothies from Smuggler Jash to restore 40 juice to everybody)

The most important skills for this fight are:
Omori: Trick, Hack Away, Sad Poem, (Stab is optional)
Kel: Run n' Gun, Guard, Flex, Ricochet (only if he's Level 16) or Juice Me (he can get this if you get a coconut during the cooking sidequest, where you can also get Hero's Frying Pan)
Aubrey: Power Hit, Team Spirit, Counter, Mood Wrecker (only if she's Level 17)
Hero: Smile, Cook, Snack Time

Use Hero's Smile to reduce her Attack, Aubrey's Power Hit to reduce her Defense and Omori's Trick to reduce her Speed (if Sweetheart is happy). Try to make everyone Sad after she becomes Happy, especially Kel and Omori. If you have Release Energy ready while your party is Sad, use it. Good luck!