I consider myself to be a mediocre player, I don't really play difficult content. My only advice really is just to practice. I think everyone has one or two mechanics they struggle with!

It took me a long time to be able to do the six half-room cleaves in The Gilded Araya trial for example, until I practiced it enough I understood how it worked.

If you can enter a dungeon or trial with NPC's you can practice as much as you want, and usually lots of fights use the same mechanics, so once you get it down you'll find it easier if it comes up again in future. Also my performance is garbage if I'm tired, so try coming back to try something again after some rest.