AD reds, flat amour yellows, AP blues, and either health or movement speed quints.
Get the AD and AP from your masteries.
Since you'll be getting both procs on your passive, start with boots and 3 HP. Then:
Hextech revolver>Sorc boots> Rylais> Hextech gunblade> Deathcap> Void staff and if the game goes on long enough, another deathcap.
While in lane, always keep poking with your ranged ability whenever they get too close, and if they come in for the harrass, proc the mark on them. You can also use your ranged attack to last hit if you can't go in melee range.
After you get your ult, you go god mode. You're not un-killable, but you become EXTREMELY dangerous to be around. Throw your mark, ult on to them, auto attack, E, rinse and repeat. Remember that CC shut's down akali hard as she is squishy and a critting GP will rape you if you get stunned or snared by a ganker, therefore. To maximize your survivabilty, throw down your shroud and then perform your combo, you'll go invisible before they can retaliate.. Gank mid whenever possible.