I have the same class makeup as you do. I also had the exact same problems with this boss. The elemental would go straight for my priest and own him. I am slowly getting better, but for me I was [and still am] having difficulty getting aggro after each phase switch. Just takes some practice [and not forgetting to drop nature resist totems!]. I was pretty drunk the last time I beat him so I don't remember what I did, lol.