This trio of big blue apes will be your first Boss Battle. They are not all that hard and barely put up more of a fight than a large group of normal enemies. The have only 150 Health each and are weak to fire. Jenna's area- effect fire attacks like Flare work very well against them. To increase your number of attackers, give Sheba the Mysterious Card you picked up give her the fire-based Juggle Psynergy. While the two girls attack the Chestbeaters, have Felix heal with Cure and attack if necessary.
If anything, this guy is even easier than the Chestbeaters. True, he is quite tough with over 1000 Health, but his pathetic attacks only did about 10 damage each! Since you should have quite a collection of Djinn by now, enter battle with them on standby so that you can unleash your most powerful Summons on him as the battle starts. Especially try to use fire Summons and Psynergies; like the Chestbeaters, KS is weak to fire. Because his attacks are so weak, it is harder to lose against King Scorpion than to win.
Your first remotely challenging Boss fight. Although Briggs the pirate and his crew are weak to every element, Briggs can summon more Sea Fighters when to down some of them. He is also much tougher than his underlings are. Because destroying his minions initially is futile, concentrate your strongest attacks on Briggs. Remember to pull off your strongest Summons at the start of the battle (concentrating them on Briggs). Since you're fighting 3 opponents instead of one, remember to keeps your party well healed. After Briggs goes down, the battle becomes an easy process as you mop up his Sea Fighters.