WHEW I just tried again this morning after writing that post and got him! First attempt I got him down to 400k health, I got knocked off the edge but Kor'vas didn't take any damage. Second attempt went almost perfectly, Velen almost died to minion damage but a lucky healing orb placement saved the whole thing.
Basically the Agatha event counts as taking part in the Broken Isles, so you can use the Lightblood elixers that procs millions of damage and the Holybandage thing that heals for 5% your HP every 3s to pretty much cheese entire fight.
I actually easily beat her on my first ever attempt at like 940 without even knowing about that method, she's one of the easiest challenges.
"It's not what we don't know that gets us into trouble; it's what we know for sure that just ain't so." ~ Mark Twain
"The time you enjoy wasting is not wasted time" ~ Jesus of Nazareth
"把它放在我的屁股,爸爸" ~ Dalai Lama
Not sure if it was mentioned already, but a tactic I used for the arcane adds was to use blood drinker on your main target once they had completed spawning to draw healing aggro to yourself. (Seemed to work even at full health?)
Blood drinker will be up each time the adds spawn, and makes it easier to collect them during the last phase.
Also I want to point out, but guides would say you just need aggro/taunt a single one of the arcane adds for the rest to follow which actually wasn't the case.
I've done it on monk with, druid, dh, dk, and... DK was the hardest for me :\
so no wonders you had troubles with blood
step into everything will gief ya nothing, mon