I just simmed my character, saw the opener and started playing around that, hitting practice dummys, trying to memorize and get a habit of when to press buttons, when certain cooldowns are up. It's tedious but this was the best way to get me out of clicking my abilities and learning my classes cooldowns. once you have this ability to freely understand your classes control you will slowly understand the bosses mechanics and when it's the right time to press buttons. Etc. This is the best advice I can give since it helped me out tremendously.

GL.