Ok, So to beat a Dragon Easier, All you have to do is move out of the way of it's attacks and while it's still attacking where you were, swing like crazy to get it good.Here are some tips for the other bosses too.

Cervantes- MOST of his attacks are avoidable by jumping or blocking. Use sophitia,Taki,Ivy,Or Lloyd

Geki And Maki- Stay Back As Far as You Can While They Are Protected. Use Taki and Sophitia

Ammon- Jump over his fire attacks Dodge the lasers. Attack him when you are close, or when his 'armor' is expanded. NO BLOCKING NEEDED. I'd recommend using a fast slow team like siegfried and sophitia.

Fafnir- Same Deal As The other Dragons, only whey he is flying, hit the dragonslayers as fast as you can. You can hit him when he is in mid attack. And you can also jump over his fireball attack.

Nightmare at sealing of soul edge- just run out of the way of his airborne attacks, then chase him down as he goes by and beat the living crap out of him. Not too hard, eh?

You can only use Siegfried to Fight Nightmare, but make sure to have him when you fight Fafnir.

Note: Charge always does more damage, just make sure you don't get hit when you use it.

I hope this helps, from The True NightMare.