This is a clear case of enrage combined with frenzy.
The enrage must be dispelled, and as fast as possible. Basically, if the horror manages to get a swing off while enraged, the dispeller failed.
The low health frenzy is much more dangerous. Cooldowns are barely enough to keep you alive, and you run the risk of having two horrors at that time. During our 25man heroic tries, I discovered two ways of dealing with it.
The first is getting all the ghouls you can get, and as soon as possible. This tactic causes the disease to stack high enough to probably leapfrog frenzy range. Note, I said probably. It's not a guarantee, and that brings us to the second method.
Stun the horror as soon as he drops under 20%. The disease will tick again within five seconds (and that should kill the horror), so most single-target stuns work, and even a 3 second stun could be enough to save you.
Those two tactics leave one scenario to deal with, and that's a horror that goes under 20% with the last tick of the disease. What you should do here is string cooldowns together (save them for this!), and let your raid know that you need help on cooldowns and heavy healing. Those things combined may save you.
I haven't yet found a guaranteed tactic to survive the horrors all the time - sometimes you have bad luck and they kill you.
Originally Posted by Kahmal
...there is no true progression for a casual anymore, just hand outs.