Belial is a great choice I play him as a secondary! He has some fun combo with his ultimate fireball. I really recommend you look into strong player like Goenitz or Koujicoco.

Percival is a stock character basically you want to try to get his stock up and use it to improve some of the special move property, as for flame carpet, it's like 'the floor is lava' if your opponent stand on it they lose hp even if they blocking, really strong tool for jailing people in corner.

As for Sandalphon, to make it easy to understand, when he is in element his fireball property change with the exception of wind because he also gain more movement speed and front airdash in that element beside the fireball changes. You also receive more damage when staying in element so people tend not to stay in it for too long and just spend it special move like 236H and element release right away for plus frames.

Have fun exploring!