I have just completed the quest and challenge for healers in the new legendary quests for 5.3. Even though my MS is Shadow, I found the Ranged DPS challenge a lot harder than the Healer challenge. Personally, I thought the ranged quest was not intuitive - you're asked to DPS behind the boss, but as ranged, you almost never do that!
Here is a video of it: http://www.twitch.tv/wowjessaurus/c/2315408
"This is definitely a bajillion times easier as a healer" is what I say at the end, but my mic was not queued for some of it, oops! Another oops: I cut the video a tad bit too soon, but you can see that Deathwing is nearly dead!
-Lightspring does not heal Wrathion
-Take DS for the adds!