First switch Chantodo's Will to the Starfire. I like Aether Walker over In-Geom for the cube.
Legendary Gems should be Zei's Stone Of Vengance, Gogock of Swiftness and Bane of the Trapped. So swap out your Bane of the Powerful for Zei's.
Gameplay - as soon as the GR starts, cast your two buffs. Then use 3 elements - your teleport (Arcane), Frost Nova (Cold), and Arcane Torrent (Lightning). Once all three are activate, then pop your Archon (for Fire) and you'll have all 4 elements and the first 8 seconds of Archon is where you deal massive damage.
With Aether Walker cubed - you can use this to teleport around and try not to get hit while out of Archon form.
Once you can deal with not getting hit and want more power, you can swap out teleport with Black Hole - Spellsteal. Gain black hole stacks and you'll deal exponential damage.