Everything in Dragon Soul is affected by the 30% reduction, including Stomp from Morchok, the adds from Yor'sahj, the AoE during the Black Phase on Zon'ozz, Pillars on Hagara, etc.
For you guys struggling with Hagara, take heart from a late February Savior of Azeroth...this fight sucks. Not so much that it is difficult...it's just annoying. RNG can mess with you in the worst way on this fight, almost to the degree the dreaded Warmaster fight has with it's (pardon my French) bullshit fire mechanic.
I've done Hagara both ways. The method where the healers and ranged stack in the middle is ok if this is where you choose to use your healing CDs. Tranq, Divine Hymn, and Aura Mastery are helpful, but I prefer my method.
What we do is I mark all 4 spots where the pillars spawn pre-fight. Blue, Green, Purple, Red. When the ice phase begins, we all group up at Blue and burst it down with no problems. During this entire phase, when someone gets Frostflake, they call out in Mumble when they are in the water sphere and need dispelled. Communication makes this phase a joke. We then head for Green right behind the now-moving Ice Wave. When we get to Green, we DPS it but I make everyone stand close to the water sphere. When the wave comes around, everyone dips in, runs past the wave, and back out to finish Green off. Rinse and repeat for Purple and Red. Dipping in and out of the water sphere requires almost no healing CDs and thus can be saved for some of the ridiculous RNG things that can happen in the normal phase. However, this strategy does make you require a high level of communication and coordination...if someone calls for a dispel early, it'll likely cause a wipe. If someone doesn't call for a dispel, they'll likely die. While most of these mechanics are easy to ignore, it's important to keep your raiders responsible for what they each need to deal with. It will cause a lot less headaches when MoP comes and mechanics are not as forgiving.
By not using healing CDs during Ice Phase, you can use them for the normal phase when Hagara makes the wonderful decision to freeze two healers right before she does Focused Assault. Or when all three of the Ice Beams start spawning from the same point and wreck one person. I hate the randomness of this fight.
Some tips I picked up in my many times teaching this fight...when an Ice or Lightning Phase is completed, make sure your ranged and healers stay at least 15 yards away from Hagara. As soon as Feedback wears off, she will begin those Ice Beams. The mechanic here is she can only target someone with the Ice Beams who is within 15 yards of her. If your ranged or healers are too close and they try to move out of the Ice Beam to drop stacks but can't because they're focused, they're either at the mercy of another person realizing this and intercepting them or an immunity cooldown. Just prevent this mishap by keeping ranged and healers far away. Also, that Shattered Ice spell? DKs can steal it and hit her with it for a nice cool 150-180k hit. Not bad for two GCDs!