Bubble to force an aggro swap + clearing your stack at the same time. Don't go super crazy generating threat on him either. You don't want to be 20k ahead of the next tank. When you're OT'ing, hunters can put a MD on you. You shouldn't be running out of mana in 30 seconds though. When he does Fel-Acid breath, try to eat that. Also if you lead off tanking and get a MD at the start, consider not using Avenging Wrath. Save that for when you OT and need to catch up.

My two cents on a few recent topics here:

"Un-uncrittability": Many of the rumors of "always being crittable" are based on someone's experience soloing Teremus the Devourer, the elite dragon that wanders Blasted Lands. Teremus has a base 10% crit rate, rather than the usual 5%. I think he's unique in this regard; certainly no known raid boss has a base crit rate above 5%.

Reckoning: Regardless of how much it impacts your survivability, I think the biggest reason most raid-tanking paladins don't take the talent is simply because they can't find the points for it. Given the large number of "required" talents in prot and the mandatory 10 points in Ret for Deflection, Reckoning tends to be one of the first corners cut. (Which is a shame, because it's really one of the most interesting and fun talents in the tree.)

Bloodboil: This probably goes without saying, but in addition to what everyone else has said, you pretty much have to chain-chug mana pots. Drink one as soon as you've burned 3000 mana, and then chug another one every time the cooldown finishes.

Well recently i skipped Reckoning. On my last Teron Fight with Reckoning skilled it was approximately 95 tps worth. Reason behind kicking it was that i wanted to take Improved Judgement with my recently looted [Tome of the Lightbringer]. So I had to find the points somewhere and well, it was reckoning. Until that time my seal was Seal of Righteousness, now i use Seal of Vengeance.
With Reckoning: SoR > SoV, without SoV > SoR... at least if you manage to kepp up die full stack

Another thing:
Redoubt vs. Imp. Devotion Aura. I can't believe someone would prefer the aura. Not only that its kinda easy to put a holy pally with improved aura in your group, but also redoubt itself is a very good talent from the moment you don't need [Libram of Repentance] anymore to be uncrushable. But the most important part is you can't skill Shield Spec without it, and this is not good -.-

My guild is very close to downing this guy, so I've learned a lot in the last couple weeks about the fight.

First off, paladin tanks are godly for Bloodboil. The ability to bubble out of the stacking debuff is a massive advantage over druids and warriors. Given this, and that pally threat is pretty weak when not being directly attacked, I've been the leadoff tank for my guild and it's worked wonders. I will tank the entirety of the first phase 1 (barring an unlucky knockback or eject), then bubble out of the 10-12 debuffs I've accumulated once he focuses someone in the raid. This means the tanks have zero debuffs active which is a major benefit to the already-overstressed healers. [Moroes' Lucky Pocket Watch] is a great tool to slow down the stacking effect as well.

BoP is amazing to force tank transitions. Communicate with the other pallies in your raid to ensure you have a rotation in place for when it's needed. Again though, don't get too much of a lead above your other tanks or else he's just going to turn back to you after it wears off. He attacks pretty quickly so HS threat adds up in a hurry; keep this in mind. When I get too far ahead I simply stop dropping Consecration and turn off auto attack. I don't feel that the survivability risk is worth dropping HS, especially when the problem is that you are taking too much damage to begin with.

Make sure your hunters always MD the tank that's lowest on the threat list. That makes transitions much easier to manage, and it provides your DPS a significantly higher threat ceiling to work with to get the boss dead sooner.

One trick I've found as alliance is to stack up SoV during phase 2. The loss of threat from ramping it up means nothing with the Insignificance debuff, so wait for phase 1 to start up again then judge it and go back to SoR. That's a pretty major threat boost if you find yourself behind the other tanks, but again, be very watchful of your threat generation so that you don't get yourself killed.

I see what you're saying regarding the other tanks, but we've tried leading with each of us so far and our best success has been with me taking the initial beating. This might be due to the fact that we run with a warrior/druid/pally tank trio, so we all get to play to our strengths. The biggest concern is that all three of us generate high levels of threat consistently, and when I spend most the fight offtanking I lose a major threat component from HS. And even with taking the initial barrage of hits I still find myself chain-chugging mana pots to keep up with the other two.

Whichever tank leads the charge is going to get a substantial lead over the others. I guess I see that it makes the most sense to have the tank who generates the least OT threat to be the first MT to get a greater overall balance. Our other two tanks are machines at OT threat, and I simply cannot keep up with just Consecration and seal/judgement combos.

I guess it depends on your setup. When I'm at Bloodboil I make sure our warrior OT has windfury, and that alleviates a lot of threat problems. Druids of course have no problem being the OT. I'll certainly jump out to a quick 10k threat lead, but I try to be pretty careful about my threat after that. Maybe the first 10-15 seconds from the pull I'll do a full threat rotation but after that I'm looking to dial it back substantially unless my OTs are right with me.

I picked up [Antonidas's Aegis of Rapt Concentration] last night from my guild's first Archimonde kill.

Now, I know it obviously doesn't hold a candle to Bulwark of Azzinoth and Kaz'rogal's Hardened Heart, or even Bulwark of the Amani Empire, but seeing as I was only using [Dawnforged Defender], I'm not sure how serious to take it. Even without a stamina enchant on it, TankPoints was telling me it's around 1,500 "points" better with higher effective health. I know not to take everything that states as gospel but, while I was at work today, I couldn't see a reason not to make it my main shield until I get something more appropriate. Considering the majority of what I do is trash tanking, 1% dodge seems a small price to pay for higher armour, block value and threat.

I guess I'm looking for someone to tell me why it's a bad idea to do this.

I switched to [Antonidas's Aegis of Rapt Concentration] from [Dawnforged Defender]. I actually have [Kaz'rogal's Hardened Heart], as well as several other shields.

I do alot of trash tanking, getting the AOE tank pulls down in Hyjal and BT. I need as much spell damage as I can get. Pulling that mostly from my Main hand and shield. For hard hitters and boss fights I throw on [Kaz'rogal's Hardened Heart].

Some people wondered why I got it. But show me a better shield for pally trash tanking

Eh, it's really dependent on the tanks in question. Our group does Bloodboil with the warrior tank taking it, and it usually takes a couple cycles before the paladin catches back up. But me (a feral) takes almost no time, and most of the time I'll have to hold back on threat so that everyone's on the same page. If you have a feral tank or two in addition to the pally, and you have a couple of hunters, catching back up isn't so hard. I really wish we'd let our pally lead; we'd have him hold aggro for a long time, which makes it easier on the healers.

But yeah, paladins on bloodboil are a big win.

It's a great idea. Gaining 1500 armor is a huge gain in overall damage reduction, especially when tanking trash in Hyjal.

What the others said. So long as you remain uncrushable, you're fine on defenses, and it's a hell of a lot more armor and block. Not only that but by the time you begin to get a lot of T6 stuff you're swimming in avoidance anyway so losing the anorexic avoidance on your shield slot won't impact you much, whereas +42 spell damage is a hell of a lot. So yeah, keep and use the Aegis for pretty much everything.

It's a damn good idea. Shields are to armor what threat is to weapons: the best slot to get a whole bunch of it. The sheer value of an extra 1500 armor (pre-Toughness, no less) is going to have a far greater effect than the avoidance losses of using a non-tanking shield.

The huge amount of armor on shields basically means that anytime you have a gap of more than 10 ilevels between shields, you should go with the higher level one unless there's a really compelling reason not to.