Exploit or not?

I was scanning the internets for my nefarious purposes (I was bored. Yes, nefarious boredom.) when I came upon this thread in the Dungeons and Raids forum. While the discussion is not particularly polite, it seems poster Selenae of Mannoroth would like to know if a particular method of killing Archimonde is considered an exploit or not. From watching the video and reading the various posts, it seems that the method in question is to cluster on a hilltop in order to avoid having to deal with Doomfires. Now, I'm not sure if it means that they get Air Bursted more this way or not... to be honest, I could barely watch the video, the cluttered UI made my eyes hurt.

Some posters seem to be of the opinion that this is 'cheesing' the encounter, while others compare it to ducking behind the pillars on Talon King Ikiss or ducking below the water to avoid spouts on The Lurker Below. So i put it to you, dear readers: when is terrain use 'creative use of game mechanics' and when is it an exploit? In the case of the Archimonde encounter, is not having to worry about the fear/doomfire combination really that big a deal? Or is the exploit inherent in the removal of the doomfire from the encounter entirely? Go ahead, discuss, mix it up, the comments await you.