Well, you could consider going for 3/3 Living Seed...
But yeah, if this is your main problem (which shouldn't be, because it is after all an ulikely scenario), use whatever you have to get through. First of all, make sure both ice blocks don't interrupt your line of sight to the tank - everybody should know where to stand if he is a target. Then your priest should use the remaining seconds to give your tank a Shield and Pain Suppression, while your Shaman should make sure the tank is at 100% hp. And then he needs to use a cooldown or two for himself, while your DDs focus on getting the other healers out. Just save Nature's Grace for this moment, stack your Hots and spamheal him.
And if you're still having troubles then, maybe try having someone taunt and kite Hagara for some seconds?
But again, this situation shouldn't occur very often.
Some different things that worked well for us: You probably already know this, but if you tank the boss at the edge of the platform, he should very rarely (if at all) use Shattered Ice. Moreover, there's oftentimes someone dying to Ice Lance, which should not happen. We set up three teams of two players each to interrupt them and assigned them to specific sides (team a takes the left lance, team b the one in the middle etc, which of course requires you to tank him at a specific location, see above). One last thing (which you probably already know, too, but it helped us a lot so just to make sure): positioning yourself like in this guide (
A Guide to Heroic Hagara [VOX] (DS) - YouTube) makes the lightning phase rather easy.