first of all try allways to kill minions from the beggining with Q skill only with Q if you got no other choice to otherwise..

when entering a battle (prefer vs AP champion because of your ulti) start with E - stun make sure to hit with it, right after use W then Deathfire Grasp Q and R make sure you make it that row, the point is to hit with the deathfire first to remove the most HP then so untill the meteor hit you should done all other skills. also point to notice is to use Q before R just incase your opponent will not die and you will need to recharge it.
If the opponent is still alive after this combo match your self to the game :)