Near the beginning of the game you can beat Balio and Sunder (not with Rei and teepo this is much harder you fight alone....or it could be easier.)
When nina gets captured by Balio and Sunder at wyndia you'll have to fight them.
If your level is high enough you can beat them(make them run off) simply attack nina she will wake up,attack sunder and run. Then change into a dragon to have enough HP.
Balio might attack then he will run same thing for Sunder.
I won't spoil the ending but you have to train beyd until he gets ALOT of everything which means you must be at level 99 without using a gameshark, and you must have last resort this is good to attack beyond if you have super combo.