This build is really strange. Bluntly: It's not a build for people whose wants to learn how to play Akali. This build is not for rankeds too (maybe on low elo). I recommend it for people whose are very good Akali players and are searching for new, interesting build.

Masteries are strange, but I've chosen them, because we need additional ad to have 2 passives at start. And we would have to waste some mastery points to reach 21 in offensive, so I decided to go 15/0/15. And 3% bonus movement speed is not bad ;)

7 flat ad runes are must be. With them and masteries in ad we have 10 bonus ad at start needed to our passive. Other 2 can be whatever you choose.
I take runes for armor, magic resistance and hp quints. They are fine for early game when we must feed ourselves :)

It's nice when you play with your friend on the same lane. I often play with Shaco, which really is a nice lanemate for killing. After 6 lvl try to gank mid to get frag or assist, but you should have at least boots of speed.

Early game
If you don't have jungler in your team, go at lane with mana golem. You can get firstblood easily at 2 lvl, if your lanemate has exhaust too. Farm minions until boots, then go back. When heading back on your lane try to kill blue golem - it will help you at farming and killing. You shouldn't have problems with it, but you can ask your lanemate for help. After reaching 6 lvl (recommended with lizard buff) try to gank mid unless it's Kassadin with flash and cleanse.

Mid game
Gank as much as you can - you need stacks. Steal frags, its simple with Akali. Try to get as much buffs as you can.

Late game
Stay behind your tank - you can't get focused. Your role is to jump in teamfight and kill carrys and nukers. Exhaust melee or ranged dps and neutralize support or nuker. Keep ghost only for won teamfights - to chasing survivors. Of course use ghost for running away, but do not use it for chasing only 1 person without a teamfight

Tips:
-Use your ulti on minions or champions to escape from ganks or lost teamfights. Try to trick 'em with your Twilight shroud - it helps a lot.

-Stay out from tank with oracle elixir - he's very dangerous because your smoke bomb doesn't affect him

-When your opponent or lane has 50% hp, throw Mark of the Assassin at him. Wait until cooldown reaches 0, then jump into him with Ulti and quickly throw Mark right after melee attack. You will deal horrible damage:
-Mark of assassin
-Shadow dance
-Mark of assassin + melee attack
-Mark of assassin
-Mark of assassin + melee attack
-etc....

If you master it, you will dominate your lane.

-Do not forget about Gunblade! This item slows your opponent for 50%! Drag it on second slot so you can use it by pressing 2.

I know: Where is Rabadon's Cap? The truth is - I don't know. Boots, Mejai for AP, Occult for AD and spellvamp, Gunblade for spellvamp and 50% slow, Rylai for slowing with each ability and for HP and Lich Bane, which gives us additional dmg when dealing second Mark's damage. There is not Rabadon, because it's hybrid build. I can't get rid of any of these items - all are must be.

With all stacks, all items and opponent without additional magic resistance you can deal 1400 dmg in one hit. Really. When foe is on Mark, attack him with right mouse button. You deal damage from mark (almost the same as your ap), normal damage (which is nice since we have occult and Gunblade), additional damage from our passive (something like additional 50% of our AD) and our AP from Lich Bane. 1400 damage? Piece of cake.

Good luck, and have a nice kills!
It's my first guide. I hope you will enjoy it :)