Here is a link to my guide for the fight which is from my pov as a whm. I have never really noticed any mp issues but maybe compare what I'm doing in the vid with what you see yourself doing during the fight and see if you can isolate where you are burning the extra mana. It's also entirely possible that its because of the way the fight is being done as there are a few ways to smooth out the damage which is easier on whm mana.

Link:
http://youtu.be/hc5Wr1uWeGk


edit: had wrong link at first