- Keep your pet on the mobs. Killing the adds is priority number one and two.
- BM has pet control issues on this fight. If you have to go BM, Blink Strike will increase your DPS. But you will still see range and control issues. Macro'ng /petattack type commands into everything can help.
- Go Survival and mow down the adds. Entrapment is decent as well. You will be happier and you'll do much better damage.
- As SV, take Lynx Rush. Damage on the boss doesn't matter much before the doors go down. Hold on to Lynx Rush until you have mobs to use it on. Yes, it's delaying DPS but it's an important delay.
- Do not blow CDs off the boss pull. Never blow DPS cooldowns on the boss until you're done with the doors. Blow them on the door mobs once each second wave comes out.
- If you have two good DPS with short interrupt CDs then they should handle interrupts and you should take binding shot and use it often -- but most especially on the third door. Otherwise, you need to take silencing shot.
- The fight is 11+ mins long. Blow Stampede on the second door when wave two and three jump out at once. Destroy the priests.
- Use Rapid Fire and Readiness often. Sadly, you probably don't need it on the first door.
- Finish off mobs that are 25% health or less before moving on to a different priority.
- If all your buffs are taken care of and you're starting to look at debuffs, then either take an extra interrupt with a Gorilla, or a casting speed debuff with a Sporebat. I did not have raid experience with hitting the interrupts via a pet, I took the auto-casting sporebat.
- AoE generally isn't worth it. Glaive tosses always are good. Explosive traps are worse than a black arrow for SV because of all the mob movement, but okay to do in downtime with MM or BM. SV multishots to apply stings are good, but you want to remove mobs one at a time not have them all up until they are all down.
- Drop frost traps in the foci of the doorway. They'll proc your LnL but also control the mobs.
The TL;DR Issues Section
First off: your pet will attack Horridon. Fine. But it will always go back to attacking Horridon after you kill a door mob. Even with another mob right there that you've already started attacking. The boss will often be positioned away from these mobs by a decent amount. So much so that BM's Kill Command, Lynx Rush and the like are out of range. You can counter that with a lot of control-1 commands or macro'ng every shot. You can also use blink strike. But it still is a problem. You can counter it by diminishing the importance of the pet by going with SV instead.
My guild runs a 10man. Horridon 10man was tuned incredibly tight last week. This week they nerfed the health of the add waves to make it more accessible to the non heroic-raiding guild. The issue with the difficulty of the fight isn't actually about DPS and mob health. But that DPS can help muscle your way through and beat down what you can't deal with otherwise. My 10man generally has 1-2 warlocks, 1-2 hunters, a mage, a DK, a monk tank, a warrior tank, and a resto druid. Rotating in are a new holy pally, a holy priest, and a resto shaman.
I'm not going into all the fight descriptions, but suffice it to say, you need to have good interrupts, good slows, and good dispel and cleansing. Each door is a different type of thing that needs to be removed if it lands on a person. If your raid makeup only has one class that can cure disease, then you're probably going to hit a wall. Same goes for poison (though it's much more common).
So with a 24 sec CD on silencing shot, we're not too great on the interrupt front. But if you run with a mage and two warlocks then you need to take it because their interrupts are just as bad. On pet selection front, if the buffs are all taken care of and you're mainly looking at target debuffs then I recommend the sporebat. Yes, he can give spell haste as a buff. But he also freely gives the casting speed debuff. And one multi-dot poison can make or break the fight.
We are decent at slowing mobs, and that's the main deal on the third door. Slow traps, binding shot (if you didn't need to take silencing shot), concussion shot, and I think scatter shot also works.
But disease debuffs will go out and we need to rely on the raid to deal with them. On our kill, I had 6 stacks of disease and was swallowing over 100k damage ticks crying for a cleanse. Here's who can do what type of dispelling:
Druid: Curses, Magic (if healer), Poisons
Monk: Diseases, Magic (if healer), Poisons
Pally: Diseases, Magic (if healer), Poisons
Priest: Diseases, Magic, Poisons (if healer)
Sham: Curses, Magic (if healer)
Other than that, a 'lock's imp can singe magic, but it doesn't look like an auto-cast. Good dwarves can remove poison from themselves for the second door. And a resto druid can Symbiosis a pally and get cure disease.
If your raid is weak on interrupts, you need to be strong in cleanses. Or vice versa. If you are in heroic raid gear you can just muscle through 10man with overwhelming DPS.
If you're doing it in 25man, there aren't extra mobs! So interrupts should be covered a few times over! That means there will be far less to cleanse/dispel. It also means you can focus more on control and nuking down individual important mobs. 25mans can also bring three tanks. From what I can tell from the dozens of threads on this, in the first week heroic and normal 25mans and heroic 10man raiding guilds got him down. A tiny number of 10man normal guilds got him down with a perfect mix of interrupt classes and healers with the right cleansing/dispelling. My guild didn't have a problem with DPS. But we hit a wall from interrupt & cleansing/dispels. And it was all just class balance.