He is not a very strong boss, just a little bit harder than normal enemies. However, he is assisted with two other enemies, which can be a big problem if they are not eliminated first.
First, eliminate the two other enemies or the future is not going to be very bright. This guy has some regular attack, and he can hit quite hard. It's very simple to deal with him. Shell can reduce magical damage. Swing will attack the whole party for about 200 damage and Body Slam will deal about 350 damage and probably inflict Confuse. The player needs somebody in the team to heal when necessary while others keep damaging him. He can't really inflict any significant amount of damage to the team. This fight should be easy enough.