Hi Soontir Fel,
Yes Earth mastery with Dream would be quite a nice build and they compliment each other quite nicely.
I have a couple of suggestion for you in the Dream tree if you are utilising Orb from the Earth tree and that is making the most of the Distortion Wave and Distortion Ripple spells. I have built 2 characters beyond 60th level now involving the Dream mastery (one char melee based and the other caster based) - I found with the caster that I built Distortion ripple and distortion wave to be very useful. Coupled with the stun ability from the Volcanic orb I think you will find throwing a Distortion wave in the direction of the monsters also will finish them off quite nicely.
Core Dweller with the Nightmare has been a proven good combo so you are onto a winner there. Just another suggestion for you if you are going down the "super pet" build and that is get Heat Shield maxed in the Earth enchantment. You can cast this on yourself, the Core Dweller and your Nightmare and it lasts indefinately so it gets the thumbs up for being worth your skill points from me.
And lastly but not least have fun with your build - there is no right or wrong way just be wary of trying to strategise a wee bit on your build so you can ultimately beat Legendary difficulty.
Have fun!
