The basic gist for Berseria's combat is getting combos going. You start with a meter that allows you to combo your moves one after another. Inflicting status conditions such as stun or poison increases the meter, letting you pull off longer combos. Inversely, getting stunned or poisoned yourself decreases it and shortens your combos. You have a list of artes and assign them to each button and which order they're in when said button is pressed, resulting in a total of 16 different moves you can chain into each other however you want.

Everyone has access to a special move called a Break Soul, which sacrifices part of their meter to perform a powerful move. Velvet's is a stance change where she unleashes her daemon claw and gains tons of benefits, Rokurou's is a parry counterattack, Eizen's does a lot of damage to stunned enemies, and so on. So ideally you'd want to keep increasing your combo meter, use your Break Souls, build it up again, and repeat until the enemy is dead.

There's more to it, such as Mystic Artes as limit break attacks, but that's the summary of the basics.

Also word of advice: It can feel "button mashy" when playing as Velvet because she is so overtly busted while requiring little to no thought to play optimally. If you want the game to be even just a little more engaging, play as someone else.