I have heard some discussion about this, but have yet to confirm it myself (mainly because I don't want to risk a run in the new version, i'm trying to unlock things).
The Angel of Death has a speed of 150%. That means that it gets to act every 0.66 seconds, or 0.53 seconds on nightmare. (I believe nightmare has 25% faster monsters?) It also has no health regeneration.
What this means is hypothetically even with no melee traits, no nothing, just your fists and your wits, you're able to clear the Unholy Cathedral. I don't know the exact mechanics due to not experiencing personally or anything, but let me try and explain hypothetically:
You have Hellrunner 2, Scout class, Agility mods on Green Armor and agility mods on boots. This will give you a movement speed of 1/((1.15+1.15)*(1.1)*(1.05)*(1.1)) which gives you a movement speed of 0.61 seconds. This is faster then the angel of death.
You move around the angel of death creating an "energy vacuum" in which the Angel loses more energy per turn then you do, eventually giving you a "free action" in which your action will succeed but the angel will be unable to act due to lack of energy. After this, the angel will have full energy (and so will you at the start of your turn), and you repeat the process. Note that (depending on how you like to hear it) the window of opportunity will be very small or the vacuum will take a long time to create due to the difference in your movespeed being only 0.05 seconds. You also have to make sure the difference is greater then your attack speed.
Hope i've helped!