Okay, now, look at Guinsoo Rageblade; it gives +% AP and AS with each hit, and this is useless. You do not have much AP, you do not have much AS(unless your ulti gives you), so what is the point of Guinsoo? Instead, buy Hextech Gunblade first for amazing spell vamp survival(also deceiving enemy champs into a fake sense of victory, a feeling which they lose after your third hit deals magic damage with your W and heals you; spell vamp is also good for using E on minion waves) and for amazing WQ nukes(or QW, if you are fast enough to hit W while flying with Q).

After Hextech Gunblade, I go for Wit's End for +mres with each hit, and good attack speed, which lets me hit maximum attack speed in a few seconds, only using my ulti. After Wit's Ends, of course, I go for Guinsoo because I need that attack damage and +%AP for both combat and pushes.