Memorization of mechanics is only part of the equation in many of these battles, and it's usually the first thing you're made aware of though experience.

I'll use myself as an example: I can be a fairly decent player. At the moment, and I won't mince words, I suck at Ravana Extreme. I've only tried it twice, but it's one of those fights I'm not consistent in yet. I'm aware of the mechanics, but when I focus on them too much, my DPS suffers because I keep trying to preempt them. When I try to output more DPS, I get sloppier with where I need to go. It's a balance of "mechanics awareness" and job proficiency, because if either one of these is off, things won't go well. And through experience, you get better at that balance and it becomes second nature.