Some good news for everyone who doesn't have an Paladin alt with iLv 540+:
Finally managed to re-kill him in 5.4.
I did it with iLv 520 - no legendary Meta, no Legendary Cloak or any else.
Pre 5.4 I had about 2 Minutes left for Enrage. I knew that it would be pretty hard now with the vengeance nerf so I prepared my alt with some DPS Stuff.
Started up with Seal of Truth till the first Transition phase and switched to Seal of Insight. Used EF instead of SS.
Used the following Glyph Setup: Focused Shield (So obvious!), Double Jeopardy and Alabaster Shield. I also used Holy Avenger for another DPS CD, and Holy Prism instead of Execution Sentence. Also consumed Flask of Winter's Bite and the 300 BuffFood and Str Potions. In addition I used the new implemented Drums of Rage (25% Bloodlust) twice: Right after the pull, when both of my Trinkets were up, and after you first leave Frostmourne.
I prepared the ghouls to die during the 1st and 2nd phase, killed them some seconds before the 2nd transition ended.
At 12,5% he enraged right after I got teleported back from Frostmourne, used LoH after I dropped to 10% HP due to the silence, popped my 50% CD and killed him.
So yea.. don't give up, guys. It is still doable.
Sadly, Invincible didn't dropped.