There is some important things you have to remember.

#1 - Last hit with your Baleful Strike (Q) as much as possible, everytime you kill someone with your Q, you'll get 1 Ability Power.

#2 - Everytime you kill a champion you'll get 5 Ability Power.

#3 - With this build, you have to be carefull early game. You dont have that much ability power in the start, because of Deathfiregrasp, and you dont have Dark Matter (W) before level 8. You may think it's stupid not to activate Dark Matter (W) before level 8, but it's actually fine enough.
You will be more carefull until you'll have it, you can farm better with your Baleful Strike (Q), because of the high level of it fast. When you're level 8, you have the Baleful Strike (Q) and your Even Horizont (E) at high level, which means that you have the chance to trap your enemies for a longer time, and then finish them.

#4 - Order to cast your spells:
- W, E,(deathfiregrasp), R, Q.


Maybe this build won't work for you, but it works fine for me, and i'll promise you, you're gonna hurt like hell!