The hard enrage seemed to be tuned pretty loosely on non-heroic. With 7 people we brought him to 40% by the time he enraged without freeing any of the NPC's. His damage did not increase appreciably when the chests were destroyed, and based on the names they had "bonus loot" seems most likely. Flash Freeze occurs roughly once every 30-40 seconds, with frozen blows about 10 seconds after it's cast. I didn't see how long it lasted, but it wasn't particularly long. Flash Freeze can be resisted as a binary effect, and anti-magic shell causes you to be immune to its effect. Divine Shield and Ice Block can be used to remove the flash freeze effect after it lands, and likely will prevent the effect from landing.
With the non-heroic setting, the encounter is fairly simple and basic. But meeting the achievement/bonus loot timers will require quite a bit of optimization. Think "You Don't Have An Eternity" with a tangible loot reward.
EDIT: Oh, one person also dodged flash freeze with a well-timed jump. It seems being off the floor is the trigger for avoiding it, hence standing on the snow piles. We didn't test levitate, but it seems like it should work. I suspect that won't be the case on live.
-Splug
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Active raid character - death knight:
http://www.wowarmory.com/character-sheet.xml?r=Illidan&n=Spyte
(usually) prot warrior:
http://www.wowarmory.com/character-sheet.xml?r=Illidan&n=Splug