Fine let me actually respond to you without joking about how i got a ban warning for saying that to someone who also complained about something on DAY 1 and not even a few hours in.
I don't know who designed him but whoever did certainly doesn't know how to actually design a boss.
The first half of ep5 was made with only hero in mind. It's very apparent and something that the playerbase didn't agree with. The result was a whole lot of balance and revisions to the older classes, and an incomplete story and quests.
He is near constantly mobile making many attacks extremely hard to hit,
Again, was made with hero in mind. However, the only real reason why he moves so much is because whoever has aggro is moving too much. If you have aggro and stay in front of him, he doesn't actually move that much. This boss also makes people use moves that are quick and short. But if you don't like it, don't worry since no one else liked it either and every next boss hardly moves.
his high health
His hp is fine.
combined with also near constant damage reduction,
Only because you aren't hitting the marked spots when it's purple. If it's blue then get the sword and break it. The only excuse for this is if your new and don't know the mechanic.
many hard hitting attacks while most them cover a wide area and even some attacks that hit you from across the map,
Learn to dodge and aware of your surroundings? He has VERY OBVIOUS tells for all of his attacks. Stop tunnel visioning and you'll be fine. But if your actual complaint is how much dmg they do... Then stack more hp and defense. Dragon was designed to have high multipliers and low attack, so defense has a much bigger impact than say ult. Also get some fire resist.
Heck ep5 and ep6's meta was to stack hp and def. It's not our fault you all didn't listen to us when we told you to get hp (and def) since the beginning.
The ability to juggle players to death with no counterplay
No bosses does this unless you are next to a wall. Not to mention air reversal is a thing. Stop walking into attacks.
broken hit boxes (attacks that shouldn't connect do anyways with no chance of counterplay)
Again, stop walking into things.
and to top it all off terrible rewards.
I agree, his rewards are shit.
I just have to ask why? Why was he designed in such a way to be anti-fun?
Maybe because you haven't even spent the time to learn how the fight works. He is a great boss that actually tests your knowledge on the game and your skills. It's a shame they decided to move to a "hit this sandbag" approach for bosses.