I think you're being too serious about the class roles in multiplayer. This is not wow where you are chopping on a dragon for 30 minutes with 40 guys and have to have everything perfectly calculated out before you start to even have a chance. Now i do realize that they said the second part of the legendary difficulty will be very tough and will need teamwork to get through. But i really doubt that you will need classes build on one role (like tank and healer). I dont think it will really matter if you're playing with a healer, tank, and damage dealer, or just with 3 dps classes. It's will be the use of skills and the good gear that will take you threw the tough part.
I dont know how good the class combination earth with spirit is, but i just tested ternion attack, and you're right, that skill does rule
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