Ok, here's my experience.
I don't know what changed but the mission I just ran (Trapdoor) wasn't the same mission I attempted for 4 hours yesterday. I looked at UberGuy's youtube video and thought that something was amiss in my experience yesterday and it seems I was right.
I applied exactly the same tactics I initially attempted yesterday and he went down before the second clone popped. Dark/Dark Corrupter (lvl 50) with a similar build to uberguy I dropped the Dark Servant, 9 reds, covered him with Tar Patch then got to work with the two Dark attacks and the snipe. He got one clone off (which I dispatched quite a bit more slowly then yesterday as it was my first run) but he went down in a few more seconds.
Same tactics yesterday and I couldn't even get the health bar to move, I tried all the tactics that have been listed on the forums (kiting/drawing, lava, ED etc) none of them made a lick of difference. He couldn't hurt me yesterday either which was odd for an EB as they usually 2 shot me even now.
In short, something looks like it changed between the two days. I reset the mission twice yesterday then gave up to do some exploring.
The one thing I did notice as different. Both times I restarted the mission (and of course every time I came back into it after rearming) Trapdoor has a full group of clones out. I killed all the clones before starting but I suspect that that may be the difference. There is no way I could have damaged him through the regen of all the clones and it could be that even though I took them all out the game still thought they were all there.
As en example, in my initial assault (yesterday that was measured by how long it took for all my red Inspirations to expire) I managed to get him own to about half health, that all just disappeared in one tick which I suspect isn't as intended. So it could be a bug of sorts.
My tip for the discussion:
I'd just keep re-rolling the encounter until you start with an empty room with just Trapdoor. If he has all his clones out, as he did with me yesterday I think that you'd have to be some kind of wonderful to beat him.
PS: Of course that means that you're going to need to fight your way down there a few times. Shouldn't be too hard.
J.