The idea behind this build is you can D+Q, E, W your enemy and by the time the second proc of Flamestrike occurs your abilities will be refreshed, and you just keep repeat this combo.

Level 1
We take Mana Addict for a lack of a better talent to fit this build in this slot. And, of course, more max mana never hurts

Level 4
We take Mana Tap for maximum mana sustain. With this talent and build, you will hardly ever run out of mana. Gathering Power is also a viable choice if you are confident in your orb grabbing.

Level 7
We take Tri-Optimal. This is a must to have your Verdant Spheres to be refreshed after you finish your combo.

Level 10
Either choice is viable.

Level 13
We take Pyromaniac. This is the talent that brings it all together.

Level 16
We take Fury of the Sunwell. For more mana sustain and max damage. Arcane Barrier could be considered as well.

Level 20
All choices are viable. But I recommend Bolt of the Storm for mobility.