Let's talk about our toolkit for a minute. Between Hand of Sacrifice and the talented Divine Sacrifice/Divine Guardian combo, we have two easy methods of redirecting some of the tank damage to ourselves. If the tank has Beacon of Light, we can simply heal ourselves through the moderate sacrificial damage, and transfer that healing to the tank as well. It's just as effective as the tank using a CD, provided that we're in no immediate danger of dying. If you've talented into improved Lay on Hands, you've got an additional CD available, though it only helps mitigate physical damage.
Using these in tandem with your tank can give them some serious damage reduction over a long period of time, which can be key for burst incoming DPS phases. Some good examples are Festergut in ICC, and Algalon (pictured) in Ulduar. Let's look at Festergut as a template for the rest of this article. Your tank will vary between getting hit with a foam bat to getting absolutely thrashed, and keeping some sort of damage reduction effect on them during the burst phases becomes key to their survival. Let your raid leader know about the tools you have at your disposal, keep them available (I have a small bar with DiSac / Hand of Sac / LoH), and communicate their usage.
You could open with HoSac for 12 seconds, followed by DiSac for 10 seconds, ending with improved LoH for an additional 15 seconds, and you've just created a ~40 second window of tank survivability. Coupled with the tank's own cooldowns and possible other external CDs from other healers, and you can greatly reduce your tank's chance of death during a burst phase. While this type of coordinated effort is best discussed ahead of time, if you know the fight is only going to last 30 seconds longer, go ahead and unload your combo to ensure that the fight ends smoothly. When it comes to keeping the tank up, you're better safe than sorry. A boss at 5% is still just as potent, and could easily wipe the raid in seconds if the tank dies.
There's really no other way to keep a tank alive in a high pressure situation than to spam Holy Light. You can try to weave in Flash of Lights with your HLs to save mana, but you're creating a possible 2.5 second or so window between Holy Lights. Holy Shock is only worth using if the heal needs to get to the tank immediately, but remember that in burst damage situations, Holy Shock will only delay the tank death by 1 second. It's great to heal the raid with, but leave the tank healing to the spell designed for it. I will typically leave the instant healing to the priests and shamans, and wind up my next Holy Light instead. Somebody needs to point some serious HPS to the tank immediately. If everyone is casting instants, the tank will get a few token Holy Shocks and Riptides, then die immediately afterwards.
Holy Light does have one downside: it has the highest cast time of any of our spells. Because it's not viable to move and Holy Light at the same time, we have to find a solution for avoiding Bad Things on the ground while continuing to cast. The answer is easy: stand with the tanks. Now, some bosses will Cleave or breath fire on you, and so standing in front of the mob is a bad place to be. However, on fights like Festergut, Saurfang, or General Vezax, you can safely stand on top of either tank, safe from all the explosions and acid splashes that threaten the ranged groups. This allows you to safely chain cast your healing spells without worrying about moving or avoiding attacks. It also has great synergy with Seal of Wisdom, since you'll be close enough to melee the boss.
Mana is life
A paladin will typically focus on reducing Holy Light's high mana cost, so that it can be used more frequently. This includes the Glyph of Seal of Wisdom and the Libram of Renewal, which are two staples of a tank healer. To keep our mana full, we've also got mana potions, we can use Lay on Hands to restore mana, and the blood elf racial yields a blast of mana. However, there are other important ways to keep up the stream of healing to our target.
Divine Plea plays a large role in our mana regeneration package, and using it during lower damage phases is the most efficient way to do it. A great example would be right after Festergut exhales all of the blight back into the room, as he loses his stacks of Inhaled Blight. We can also use Divine Plea during medium damage phases by tying it with a healing boost cooldown like Avenging Wrath or Divine Illumination (with 2pc Tier 10, of course). I like to wait until a phase where I'm casting quite a bit and my mana is lower (less than 50%), and I'll use Divine Illumination and Divine Plea together. I end up with the 25% mana from Divine Plea, and since I was casting pretty often, Divine Illumination saved me 50% on each cast. Many paladins make the mistake of leaving their cooldowns idle: use your CDs at any helpful point.
Similarly, we can proc our Seal of Wisdom by meleeing and judging our target (Shield of Righteousness also procs SoW!). Restoring mana during low damage phases and then burning that mana when the damage is high is the key to any successful encounter. Mana earned early is mana you can use later, and it will ensure you don't run into the brick wall of 0 mana, which typically ends with a dead tank. The key of paladin healing is that with enough mana, we can keep any tank alive through just about anything. Managing our mana properly is just as important as our spell selection when it comes to our tank surviving spike damage.
Finding the right times to use Divine Plea and our other mana regeneration tools will become the test for a paladin healer on any difficult encounter. Every fight has moments of peace and moments of chaos, and balancing the two will allow you to keep the tank alive against incredible odds. Always be searching for even a small lull where you could get a few ticks of Divine Plea or throw a few melee blows in to proc SoW. You never know when a fight can go south, and having the mana to handle unpredictable events will make the difference between success and failure. Abuse your cooldowns as often as possible, and leave no stone unturned when it comes to optimizing your mana and healing management.
With our extensive toolkit for dealing with tank damage, paladins are clearly crafted with the tank healing role in mind. Beacon of Light allows us to replicate that powerful healing to two tanks at once, and makes us invaluable in raid environments. However, if we allow a tank to die, then the wipe is on our shoulders. With great power comes great responsibility, and so using every available cooldown and ability on every major encounter ensures that we don't disappoint our raid when the chips are down. It's better to be safe and cast a Holy Light than to be sorry we tried to save a bit of mana and let the tank get smashed.