I have never once survived a Frost Blast.
The reason is that there are three ways to heal Frost Blast, and most healers I've met use one of the two wrong ways. Yes, of course there were other factors involved, and some bad luck too. But most of those times I was the only FB target, and there’s no good reason anyone should die to this when it doesn’t chain to other folks.
Method 1: “Help me I'm ice-blocked!”
Someone yells out on vent. Either they're smart and give you their name, or you know their voice and spend a few brain cycles connecting voice to name. You then hunt for the name in your raid frames and do your healy thing.
You're subject to several types of delay, the first being how long it takes someone to shout, after they recognize they've been ice blocked.
Method 2: DBM
DBM will put up a raid warning indicating who's been Frost Blasted. You then follow the same thread as above: you hunt for their name on your raid frames and heal.
Hopefully you can find the DBM announcement very easily in what is likely to be a cluttered screen.
But that's not the main problem, really. The issue with both of these methods is the time it takes to find someone in your raid frames. When you have 25 people, you probably know exactly where to look for about 5 of them. All the rest? You're gonna be hunting. With a hunting system, you have maybe a 50/50 chance to save somebody, maybe a little more. But two Frost Blast targets at once? No chance.
Method 3: Now with preparation!
After we killed Sapph in our last 25, I asked in vent if healers had set up their interface to show Frost Blast.
“What!?” Our tree-druid officer practically exploded with indignation that I would be telling her how to do her job. Whoops!
This is a reaction-time fight for sure. And if you're waiting for DBM to tell you who to heal, you've gimped yourself severely, and people will die. If you're waiting to hear someone say it on vent, the total time between Frost Blast and your heal is even greater. Sure, you might still save them, skin of their teeth, but you're still not performing your job at peak capacity. You should be able to easily heal two people through it.
Before I set foot in KT's chamber I read up on the fight and realized I needed to prepare in order to succeed.
What I did is simple, but helps a ton. I’m almost always the first to get off a heal, and I’m working on being able to reliably save three targets. (I’m not there yet.)
Presto. In my raid frames I’ve colored the Frost Blocked target purple. There is no mistaking who needs a heal. There is no need to correlate a name with a raid frame. The purple color is like a magnet; the only delay is the eye-hand reaction time of standard whack-a-mole healing.
You can easily set up this sort of custom coloring in just about any unit frame addon, from XPerl to Healbot. Just do it! It saves lives!
Oh by the way, what spells to use?
Very simple. If you have one target Frost Blasted, hit them with Penance. This is the only fight I can think of where I reserve my best spell for someone other than the tank. I'll use Penance on the tank maybe once or twice after a Frost Blast, but after that, I'm saving it for the ice-blocked victim.
There are good arguments for shield+Flash Heal, and that’s totally fine. Basically anything except Greater Heal will work.
If you have two targets, Shield A, Penance B, Flash Heal A. You can even cut off the third tick of Penance if necessary. Heck I don’t know – if you’ve set up your raid frames for visual notification, you should have plenty of time to save two targets using just about any combination of spells, assuming they were topped off before Frost Blast.
For three targets...just hope you get lucky and your other healer(s) pick the target that you skipped. You can start playing around with using Prayer of Mending / Shield / Penance to try to save three targets, but like I said, I haven’t quite found the best rotation for three yet.