1. Bone storm build has been nerfed, the main damage from that had come from mirinae gem but that was patched, switch that gem for the esoteric gem for more defense or bane of the powerful to kill elites faster. Also the unity ring is a must for that build if you want to be near invincible.
2. Your rathma build (glass cannon) should be able to clear gr75. Go to the screen where you can change equipment and hit the triangle or square button (i do not remeber which, should be above the x button if i remember correctly) write down your bonus damage to elites, critical hit chance,critical hit damage and cool down reduction for both builds. You want critical hit chance to be above 50% and as much a critical hit damage and bonus damage to elites as you can get.