A combined group of our two raid groups downed Horridon the other night. I wasn't expecting to even raid at all, really. Every week there's a raid scheduled titled TOT/TOES/Something, and this week that something was success!
I have improved my add-killing ability significantly, primarily by switching to Marked for Death (MfD). I had been using Anticipation, since it's combo point efficiency is amazing, but that's a moot point when targets die before 5 combo points can even be built up. Using a few combo points on Rupture on a few adds along with plenty of 5 point Eviscerates from MfD increased my DPS significantly. I got this tip from Icyvein's Rogue specific ToT guide.
I've also started doing random battlegrounds. There's quite a difference between killing adds on Horridon and killing other players in PvP, but I've got a gut feeling that being good at one will help with being good at the other. I'm probably not going to go all out with rated battlegrounds or arena. My intention is just to hone my skills.
In retrospect, I think that playing in PvP is one of the activities that has consistently helped me master any given class. I very quickly became a good Hunter in Wrath, but I didn't get really good in the sense of using the full tool set of the class, mastering movement and survivability, ect. until I'd done some random battle grounds other than the occasional Wintergrasp faceroll. Learning how to jump-Disengage was a very handy trick that has kept me alive in raids ever since.
I started raid healing on my Priest in early Cata, and I was good at it, but after doing rated battlegrounds, I gain a lot more positional awareness, by which I mean the ability to visualize the battlefield and where other players were (specifically the tanks), how far away from me, how far away from each other. Being able to hypothetically close my eyes and point to where I knew someone is added a final polish to my ability to keep tanks up.
Currently, my tough nut to crack is how to handle the melee of adds. Switching a talent point gets me halfway there. I didn't think to get a screen shot of the damage meter, but I'm sure it was good all around. I'm also sure there's still room for improvement, and that means increasing my time on target. That itself means I need better movement and positioning.