Typically a boss fight has 2-3 phases. Phase 1 is the hardest and longest part usually that really tests your patience. Phase 2 usually begins 50-60% into the battle and you really start to distinguish the pros from the noobs. Phase 3 starts at 80% and is called the 'execute' phase where you're trying to burn the boss down with your best spells and abilities as fast as possible before the boss burns you.
I say Phase 1 is the hardest because that's the part where people lose morale and give up, stop showing up because of the grind required to master the boss until you defeat it. Phase 2 and 3 just naturally kick in once you got the muscle memory down. Phase 2 and 3 is the part you realize how much you either suck or are good at something.