-( Story related, cannot be missed )-
Fern is your first real boss and the first real challenge, too. He will be buffed by his comrades, meaning it would be in your best interest to try and debuff him with Bunk effects early on. Jake's tornado attack gives him a chance to cause Boggle on an enemy, lowering their accuracy. Coupling that with either Finn's chance to Freeze or Marceline's chance to Overheat will make it much easier. Or you can always Flump his defense and make your hits more powerful. Don't be afraid to use your Specials, too.