Oh this one is fun. There are 2 ways we've done it.
1) The way it is intended. He will randomly target one of the six platforms to do his fire breath at, There are two important checks for this to get the achievement..
- Everyone who was on the platform must run off it before he does his breath
-His breath is a AE rain the is directed to the platform. It will rain for 3 to 4 ticks (18-24 seconds). No one can run back on the platform that he breathed at during that time or they will get hit and fail the achievement that way. Requires everyone to be on point and understand what platforms they are to go to, and what to do when a platform is hit with AE.
2) Load him up with every single dot you can before each of his flying phases (90, 60, 30). Have everyone run and stand only on an egg platform, and let the dot's do your work for you. Any dot that is on him when he takes flight will count, however you are only able to cast on him while he is in the air from one of the 6 platforms that he blast with his breath. I believe rangers can bow him as well from anywhere. We fully dotted him and then ran to the egg platforms and let him tick down on his own each phase.