I run Lost City as a part of my tome farming routine, but I can't even begin to describe how many times we get to Diablo and people REFUSE to open doors or how some groups will constantly wipe to the second Ruinous Omen.
The two most common excuses I see are:
-"I'm not good at doors..." (wtf does this even mean?)
-"It's not my job. I need to heal/nuke/ranged DPS." (Orly?)
So let me explain how stupidly simple the doors work because if this lolMNK can figure out how to open the doors correctly anyone can open the doors:
When you start the Diablo Fight, the doors will open and reveal what they are. Basically there are four matching symbols/colors:
Blue -Gobbue
Green -Butterfly
Red -Ahriman
Yellow -Lion
After a short while, they doors will close and the symbols will no longer be visible. So either memorize the matching doors locations from each other or mark them. I would recommend marking them like this as an example:
1 AND 2 = the two red doors (1 and 2 represent a pair because we can't mark things with the same marker, ie can't mark doors as 1 and 1, 2 and 2, etc)
or
Square AND Triangle = the two blue doors
or
1 and 2 = red/ahriman
Chains 1 and 2 = yellow/lion door
So let's go with the first example. If the blue gobbue door is marked as "1", you open it after the fight starts and the doors are all closed then find where the door marked "2" is at. Once its location is established, go back to nuking, healing moarly, DPSing harder or whatever.
At this point keep an eye on two things: The location of the second door that MATCHES the first door as it should be marked with a "2" and Diablo himself.
A message will flash across the screen which is the first warning that Ruinous Omen is coming and Diablo, like Acheron in CT, will take the center of the platform and start casting Ruinous Omen. This is your cue to run your ass or get to closer to door marked "2".
Timing is everything!! If you open the door to the void too soon, you will be teleported to early and eat Ruinous Omen. If you open the door too late, then you're just hit with Ruinous Omen. And just so you know, most well geared players can survive the first Ruinious Omen, but after that you'll notice there are 2 or 4 stacks (can't remember exactly) on you and if you get hit with Ruinous Omen again it's pretty much a guaranteed wipe.
When Ruinous Omen hits about 45/50% open the second matching door marked with "2". If timed correctly, it will teleport you and your group across the platform to another door/portal bypassing Ruinous Omen for 0 damage. After that, go back to doing whatever but make sure to keep an eye on the door again and basically rinse and repeat.
Hopefully this makes some sense. Please leave comments or thoughts if I've missed anything helpful or if you have any tips.
Good luck!