Until seeing the video that Kinch posted, I didn't realize I could dodge the fire before it was cast - nothing is said in the original video about dodging it, so I stood still and took the initial hit rather than risk aiming it at someone. If you do take the initial hit, you will always get popped up into the air and will take a large, but survivable amount of falling damage. The last phase will be much easier for my healer next time :D
If you are getting knocked into the air then you are running into the white(ish) orbs that appear in the room during that phase. They can be really hard to see with all the fire etc.