This build is only semi-serious to be honest

If you want a more effective build, remove one of the Gunblades, add a Guinsoo's Rageblade and replace another Gunblade with the Lichbane.
Also switch Boots of Swiftness for Mercury Treads, Sorcerer Shoes or Ninja Tabi if you wish

But to the build at hand
The runes are chosen to give you ability power as you go up levels and magic penetration is obvious
Dodge runes are obviously to combine with the dodge mastery to give you a larger dodge chance at the game start (nice)

Why not have base AP runes you and rape at the start?
Because you only want to lightly harass and make some stupid and obvious mistakes at the start to lure your opponent into a false sense of security.

At level 6 get your ultimate and rip them to shreds using your Q spell and attacking and throwing down your shadow bomb to slow them. Continue to slice away and throw another Q spell or E spell and you should quickly net your first kill.

Your ultimate is really what makes you you

You Q spell does damage and damage again when you hit meaning your burst damage is massive even at level 6
Jump at an enemy. Use your Q, E and use your Gunblade/s active ability/ies and quickly tear down your opponents health
If you ever get in real trouble throw down a shadow bomb slightly further than flashs reach from a bush. Then flash toward the bush and quickly move toward it. You should remain invisible for a half second allowing you to enter the bush un-noticed
Or just use your ultimate on a far away enemy minion if any are nearby

This build hasn't covered much of Akali's power to be honest but I was just wanting to put it out there to see what people think of the massive power you can get from combining your abilities, the gunblades active and ignite together.
I've had great fun with it so you should too

Just use the rotation R, Q, attack, E, gunblade, attack, gunblade, attack, gunblade, R, Q, attack, ignite and if they are still alive then you are fighting Cho'Gath

Note: That you should time the use of your Gunblades so that you use it once the last Gunblade shot wears off. This slows them for the maximum amount of time as it is a unique active. Although I believe the damage still counts so pressing the item hotkeys (1-6 depending on its slot place) you can unless a large amount of burst damage quickly

The real might of this is that the Gunblade is constantly slowing your enemy combined with Rylai's meaning your enemy will never escape you

Another Note: Teleport is also a nice ability on Akali. Means you can teleport out of trouble once you have used your smoke bomb to escape part way. Other good ideas are cleanse, ghost and exhaust for escaping, chasing and fighting
If you also have any better runes for Akali let me know (apart from all ability power runes obviously cause thats nothing original)