The nerfs that made phase 1, 2 and 3 much easier really lowered the difficulty of the boss.Bregor said:You've now killed the hardest boss in BC (in my opinion). All the bosses in BT and HS are easier than Kael.
I have to say, in my opinion, that Reliquary of Souls is the hardest boss. It's the most complex and least forgiving boss of all.