So Flash is nice for engages or disengages alone with the increased move speed from that passive that almost always guarantees your escape/kill if you utilize it well. Ignite is very good with AP MF because most of your early game damage comes from Liandry's and your e DoT, so if they're low and getting burned by Liandry's, why not just throw a little more DoT on to secure the kill? ;)

There are a lot of questionable items like maybe a Zhonya's or a Hextech Revolver would do me a lot of good. Please, just no. All the items listed make sure that you can instaburst almost anybody. Guaranteed on squishies and maybe even some tanks.

The health on the green bar is inaccurate because it does not consider the Rylai's Passive over 20 minutes. You should have almost 3k hp with about 1800 Mana.

So you want to max your e because it's your highest scaling AP skill and it does a ton of harass and a simple e can secure your entire wave of farm allowing you to harass your opponent and deny them farm while laying out a ton of damage on them. A full combo would be to E to Make It Rain, quickly follow up with a Q to shoot a big Double Up right in their face and then R for some Bullet Time action. Afterwards, pop that w and finish off any stragglers left behind from their team's decimated corpses.

I suggest that 21/9/0 is the best option. It gives you a ton of damage, MR and health, and then even more damage.