Our raid team has been struggling on Horridon the past 3 weeks as well.
Week one: Was just crazy, all of us screwed up everything
Week two: We got our Interrupts and Dispels squared away but people were standing in the poop, so we kept dying
Week three: Interrupts, Dispels were awesome, no one died to the poop, but our DPS sunk way low. We went from averaging 85 - 90k to averaging around 65 to 70K. Adds would overwhelm our group, add tank would start losing new adds to the boss tank and we'd die on the 3rd gate.
It's a learning thing, I know. We'll get her eventually, We're a casual group (5 hours of raiding per week) but not lazy. Horridon should have been the 4th or 5th boss. Seems like a huge jump up in difficulty going from Jin'rokh to Horridon. I know the fight was nerfed a few weeks back, but on a big raid like this, when you have lots of mechanics to deal with, it would be nice to have some flexibility as far as DPS is concerned.