OK so I've played three times, two of which I won first place, and the second I lost but still received first place on my team. I don't know if this build is that great or not, because I have not had the chance to fully try it out myself, but in my mind it seems like it could destroy.
Masteries are 9-21-0 with the 9 being into the 15% MP, and the other 21 are for tankiness/added AP. For runes I'd go 3x Movement speed Quints, 9x Mage Pen Marks, 9x Dodge Seals, and 9x Stack Mage Resist Glyphs. If you think thats too much Magic Penetration then go for some flat AD Marks for early game health and damage. For summoner spells go for ghost and exhaust.
When you start you get all 3 skills. From there max Leap Strike first, getting Counter-Strike in the in betweens and maxing it second, and then Empower last. Of course get your ultimate at every available chance.
Start the game with Ninja Tabi and one of those yellow books that give 20 AP for 435 gold (don't know the name ), and from there build a Hextech Gunblade. Now if the team is heavy in magic damage go with a Wit's Edge, but otherwise go for a Malady. From there build another Hextech Gunblade for extra life, damage, and survivability (of course, don't forget to spam your skills or else this is a waste). Then, if the enemies are AD heavy go for a Frozen Mallet followed by an Atma's Impaler, but if they are caster heavy go for Rylai's Crystal Scepter followed by a Lich's Bane.
This should provide you with at least 3k health and so much life steal/magic resist that it'd be difficult to die. You should also dodge at least once every 3 hits, which will help protect from AD enemies. Last, you should move relatively fast to help protect your turrets/capture them, aid allies, and prevent those sneaky enemies from getting away. Maybe if you'd like added late game movement speed, and the game is for some reason still in play, switch out your Ninja Tabi for a Trinity Force, or some other quicker pair of boots.
Feedback is appreciated.