Okay, so this isn't exactly breaking news. Heck, you can find all of these strategies out on Bosskillers or probably a dozen other raid strat. sites. But I'm going to talk about it anyway.
The fight is the Four Horseman. They basically all have to be "tanked" or have somebody in their aggro range or they'll do mass damage to all players in the room, essentially causing a wipe. I'm not giving details here, so if you want the actual strategy, go check out on the Internet. Another "basic" of the fight is that you can't stay near one of the Horseman for too long because they give you these marks that will cause damage and they stack. Generally you want to have no more than 4 marks. "Switches" seem to begin at about 3.
So... the strategy that we've been using is essentially have 3 people "tanking" Lady Blaumeux and Sir Zeleik in the back while the rest of the raid splits to tank Baron Rivendare and Thane Korth'azz with traditional tanking. The 2 Horseman in the back will stay in the same spots throughout the entire fight. These are the two you have to worry about mass wiping the raid if nobody's back there. The two in the front are punks, but of the two, Thane is more dangerous with his Meteor spell. Like I said, we split the raid and took out those two first, the continued on to the back 2. This strategy is pretty much the essential strategy for beginning Naxx, or lower raid DPS.
Primogeniture doesn't have low raid DPS.
For some reason, we kept having issues with this fight. Generally, it was 1, then 2, then all 3 of the "tanks" in the back. (Generally 3 healers.) Maybe we just left them back there too long, I don't know.
So, we changed to 1 tank and one healer on Rivendare (didn't we kill that guy enough when farming our blues back at 60?), the 3 in the back, and then the rest of the raid on Thane, basically making his Meteor kind of tickle and nothing more. (~52k damage split between 20 people is only about 2600 damage.) Pop all cooldowns including Heroism (or Bloodlust), and kill him before you get too many marks. I think we killed him just before 4 marks. Then head over to Rivendare, slap him upside the head, and then head to the back to finish them. Maybe it's just me, but somehow, that was way easier.
Morale of the story: if you have the raid DPS, focus, kill one at a time, and then the 4 Horseman are easy mode. This is, of course, assuming you're not going for the achievement. If that's your goal, go check out somebody else's post. (link is 10-man version)