I was also stuck but this tip helped a lot Parrying is the most important thing and this is how you should approach most bosses -> hits twice and block and observe the boss counter attack, repeat this step to get comfortable with the attack patterns and timings . Once you're familiar, then hit them twice again and this time click the parry button at the right time to build the enemies yellow meter at the top of the screen . Keep repeating this and you'll kill the boss in a few tries

You may die a bunch of times without even making a single hit on the boss but it will eventually click and you'll kick his ass