Buy some Dark Coins in Baizel, those will always help. Attack him when you can, heal when he's flying around. They will use some dark magic on you. If you have Saint Beam, use it. Otherwise, attack with weapons.
Vacation is over, prepare for doom.
From now on, final bosses will be different, for each scenario. If the final dungeons, you'll meet some stronger versions of bosses you've fought before. They are not on this list. This battle will, of course, be extremely tough. You also have to defeat the Bloody Wolf and Camilia Queen.