How do you remember every instance?

[Chiramu]

Because most Boss fights can be broken down as follows:

1. Avoid AoE
2. Kill Adds
3. Kill Boss

That is the typical priority, though that can shifted around to ignoring AoE, ignoring adds, etc etc. Most added things are minor for the most part, and as others mentioned just comes down to muscle memory.

With gear you can pretty much skip adds when the boss at 10% and lower HP. I get pretty upset seeing people switch off the boss when you can kill him before the next add wave ><.