A total of 57 points in the mastery bars is probably not enough for level 85. I’d aim for 80+ points. On the other hand there is a pure Demolitionist build, but I think he compensates with equipment.
I doubt the Iskander’s Elemental Exchange line and Elemental Awakening will help you that much, due to the reduced weapon damage of Frenetic Throw. Likewise Star Pact won’t help you much. Instead you want to have Mental Alacrity.
For picking constallations I look up their stats on http://grimdawn.gamepedia.com/Constellation