What make HiFI Rush so easy as a rhythm game is that in order to stay on beat, all you have to do is just hit the attack button after your move lands. All hits land on beat no matter what and staying on beat no matter which attach you choose works by just hitting the attack button when your hit lands.
So, if you are off beat when you do your first attack, when it lands it will be on beat and from there just hit the attack when the first attack land to stay on beat. The other part that helps within HFR is that under your character feat it will light up when you need to hit the attack button. This helps for when you are using attacks that skip a beat or timing the bigger bonus moves using the cat. The game is very well made and if you had a bad experience playing HellSinger, this game is way easier to stay on beat without having any real rhythm.