I think it's hilarious that three times you just tried to fire off a simple DPS rotation without any reflection on how you might approach the fight differently.
Yeah, people practice rotation on dummies and then they expect that enemy will be behave like dummy
I destroyed this boss some time ago on stamina dk (when stamina dks were really poor dps), it was all about running and avoiding his attack.
This is the reason I never work with dummies, I learned a lot just by playing game and facing different eniemies and situations. You must to know your class and build to know exactly what to do in every situation. This is why often in PUG dungeons or trials I have good enoigh damage but also survability. I'm almost always on move, aware of soroundings, kiting bossess, etc. so when standing still players dies, I'm still alive.
We should have more challanges like this, then maybe players will learn how to fight