The best time to do attacks is generally while you opponent is entering a more difficult sequence. It's easier to distract them then and harder for them to think to dodge in time. Keep in mind that when you use an attack you must still recover yourself by not missing the next move you were supposed to input. The same goes for your opponent if they succeed in dodging. Once a player is experienced enough to consistently dodge accurately, attacks tend to become somewhat obsolete, so use them at your own risk and best judgement.
High scoring moves can be used only during the character solos. If you know the first sequence for a given character's high scoring moves, enter it at the beginning of a solo. Then the rest of the moves will be displayed to you. However, bear in mind that if you cannot complete the entire sequence it will do you more harm to good. Therefore there are times when it is better not to use them.
It's possible to do moves out of sequence when not dancing through a character solo. It will depend on the level of your "heat meter" how
complicated a move you can pull off. Keep in mind that you will score the most points for doing moves in the sequences presented to you by the game, but perhaps there's a way to use this trick to your advantage.