harrass with reg attacks and malefic, as soon as they think they can stay with less then 50%, flash over the mobs throw ur null zone under em hit em with ur ulti and ignite (or of it looks they'll die, dont waste the ignite)
my spam is usually W under them, or ahead of them at a choke point if there trying to escape, then Q then E then R on the zone with a possible ignite at the end....altho, situations differ, so do what u have to do.
my item setup goes like this
dorans ring for start
rush a rylais, most likely the belt or rod components 1st
if the situation calls for it, stop 1/2 way through the rylais and get a pair of stage 1 boots
after u finish those 2, ur pretty much on ur own, since the game dynamics have changed hard by that point (usually around the 16-20 min mark)
w/e u do, just remember that as soon as u hit 6, the balls in ur court.