While your DPS plays tetherball in the back with the shards and your healers feverishly try to gather enough stacks to allow them to heal through the beating you're going to be taking on their shift, you'll be tanking what boils down to Patchwerk on steroids, with three different phases that he bounces between.
The two tanks facing down Baleroc have two distinct jobs. One will be on Baleroc during his normal melee and the Inferno Blade phases, while the other will only tank during the Decimation Blade phase.
When the fight starts, the tank who is tanking Decimation Blade phases charges in first and holds the boss until he has two stacks of the Blaze of Glory buff. When that occurs, the other tank announces he's taking over and taunts, holding Baleroc until the first Blade phase (which will follow soon after) and praying for two stacks of his own, just in case. The reason the Decimation tank wants two stacks of Blaze of Glory to start is because with at least 180k HP, he'll be pushed up to 250k HP with the two stacks of the buff, thus reaching the minimum amount of damage that Decimating Strike will always hit for.
Normal melee phase
Baleroc just beats on his target, doing physical damage that starts at around 60k and increases further and further as you stack more and more of the Blaze of Glory buff. Thankfully, our block mechanic is beastly good for these, allowing us to shave off 31% of the damage of each strike (provided you actually block). I cannot recommend enough pushing your CTC as high as you can if you're the melee/Inferno tank.
The vast majority of the fight will be spent watching the melee/Inferno tank getting his face pounded in by Baleroc's fists of fury. The tank will continue to stack Blaze of Glory while the boss continues to stack a buff of his own, Incendiary Soul. This will come into play in the potential blade phase I will detail next.
Inferno Blade phase
When Inferno Blade is activated, Baleroc will put away one of his swords and charge the remaining weapon with a fiery aura. For the next 15 seconds, he will strike the tank with melee attacks that deal an additional, painful fire damage component (further buffed by that Incendiary Soul buff he's stacking on himself). This results in what is basically a magic attack yet is subject to avoidance and other defensive mechanics. You can dodge Inferno Strike; you can block it, and you can resist and absorb parts of it. It hurts, but you have a lot you can throw against it.
As such, to counter Inferno Strikes, you'll want to immediately pop a glyphed Divine Protection to start shaving 40% off the top. Because Baleroc seems to cast Inferno Blade about every 50 seconds, it's wise to also have the Mirror of Broken Images equipped as well, and use the clicky on that trinket alternating with Divine Protection. One should always be available.
As you get toward the end of the fight, all the buffs flying around on either side results in some truly gigantic numbers for this phase. Look at the very last Inferno Strike I took on our kill this week:
[22:41:53.939] Baleroc Inferno Strike Rhidach 467141 (A: 98738, B: 254236, R: 227810)
If I didn't block, absorb, or resist anything, that would have been 1,047,925 damage from that attack. It boggles the mind.
Decimation Blade phase
The most worrying part of Decimation Blade is that it is a mechanic that puts your survivability completely out your own control. Every time Baleroc hits you with a Decimating Strike, it hits for 90% of your health (and a minimum of 250k HP), and no damage reduction effects will shave that down to a happier number. On top of that, you can't block it; you can only hope to avoid it -- again, out of your hands, thanks to RNG.
However, there is one bright spot. If you're the poor, benighted fellow charged with eating those attacks, you can use Ardent Defender as a cooldown to reduce that 90% damage. The reason AD works is because it is technically a damage absorption effect, like a discipline priest's bubble, and thus not subject to the same restrictions as a damage reduction effect. So, for 10 of the 15 seconds of Decimating Strike, you can absorb 20% of that incoming 90% damage and have a Get Out of Jail Free card, to boot.
So to mitigate the unmitigatable, you generally have to rely entirely on your healers. It'll be up to them to heal-bomb you back up to the safe zone of more than 90% of your health. Likewise, you and your co-tank should absolutely work out a system where if one gets hit, the other taunts off ASAP to allow for the usual Decimation tank to be healed to full. Then take the boss back. This will severely cut down the possibility of a tank getting murdered by two hits in succession. Teamwork is essential!
Keep in mind that Decimating Strike, when it connects, puts a debuff on you reducing your healing done by 90% for 4 seconds. So if you've recently be hit, you can't slap Lay on Hands on yourself or the other tank until the debuff falls off.
Moreover, dodging is a huge help with Decimating Strikes, since avoidance works against them. You should consider using Vial of Stolen Memories as your second trinket. A huge burst of dodge can be a lifesaver going into this phase. And with a 20-second duration on the clicky, you'll have it up for the entirety of the Decimation Blade phase.
... but don't get broken down
When all is said and done, Baleroc is definitely an intense fight, but it's also one that can be very enjoyable if you're somewhat of a masochist and love a good challenge. Be prepared to die -- a lot -- as the healers work out their rotations. Your life will feel even more in their hands than usual, because it will be. The damage comes hard and fast, and more often than not, you'll find yourself running through the battle rez cap just by yourself. The key to success, it seems, is to keep throwing bodies at Baleroc until he reaches his preprogrammed kill limit and is forced to shut down.
Good luck -- and tell your friendly neighborhood repair vendor I said hello.
The Light and How to Swing It tries to help paladins cope with the dark times brought by Cataclysm. Check out our protection 101 guide and our suggestions for protection paladin addons.