Hello everyone, this is Abnormal Beats a.k.a. xAbnormalx99 bringing to you a guide for Akali. Since this is my first time making a guide please please please dont expect much from this. Also if you could give some constructive critisism it would help as well. Anyway, I hope you guys find this helpful.
(I apologize but I really suck at using BBcode)

As far as the runes go, I would recommend Greater quints of AP x3, Greater mark of magic pen x9, Greater seal of armor x9, and greater glyph of scaling AP. Firstly, I would recommend Greater quints of AP because you can start out with some damage output or else you would not burst as much early game. Secondly, I would recommend Greater mark of magic pen because if the enemy team builds MR, You can be like, LOLNOPE I got magic pen and still burst them down. Third of all, I would recommend Greater seal of armor so you dont have to build armor because with these seals your armor is handed to you on a silver platter (not really...because it costs IP). Lastly, I would recommend greater glyph of scaling AP so you can get a **** load of AP late game so your burst doesn't fall too behind.

Now, for masteries, I would recommend 21/9/0 because it benefits your burst potential and as well as your survivability. Thats all for masteries I guess.

Sorcerer's shoes allows you to add to the magic pen you already have and make it so that their MR items have no use.

Hextech Gunblade allows you to do additional damage aside from your burst and secure kills and also slows your target so he/she is easier to chase and kill.

Rabadon's deathcap is essential to your burst kit. It gives you a ****load of AP and a 30% current AP boost and allows your AP to skyrocket and make people die instantly.

abyssal scepter allows you to have MR as well as some AP. This helps because if the enemy AP carry is doing massive burst damage to you to can have some MR to defend against that. It also has a passive of decreasing the enemy MR so you can have a better chance of bursting them before them bursting you.

Deathfire grasp allows you to do percent damage which is good on champions with lots of health, like tanks. it grants the same amount of AP as a Rabadon's Deathcap but doesn't have the unique passive of increasing your AP buy 30%. It will, however increase your chance of bursting down tanky champs like Garen

Lich Bane can be used for extra damage for your auto attack. for example you used a skill and then you got silenced, what to do now? If you have lich bane you can easily use lich bane's spellblade and still apply damage even without your skills.

Zhonya's Hourglass is a very useful item. It gives time to let your skills cooldown, denies some skill shots and saves lives with the unique active. It also gives a ton of AP. It's a must.

Void staff can be used if the enemy team has lots of MR because this item cuts straight through the MR. It also gives a decent amount of AP considering that the item gives 35% AP pen.

warmogs armor Warmog's is very situational. It's something I don't really recommend getting because I use Rylai's Crystal Scepter for my survivability and AP. Anyway this item gives your the massive amount of health and HP regen that you might need for teamfights, survivability, etc.

Always start with Mark of the Assassin just for early game burst and poke.

The reason why I get Twilight Shroud second is because you can easily throw Q, then throw down your W and walk up to them to proc the Q. Anyway it's all a matter of preference.

Flash is useful just in case you run into a situation where you cant escape with your skills so you can clutch escape and save yourself from dying.

Ignite is useful for securing and reducing health regen. You use it when someone is low on health or when someone, such as Dr. Mundo uses an ulti to regen health.

Pros

High damage output (Burst)
Can be a bit tanky
Has good escape mechanisms

Cons

Gets crippled by

Vision Ward or Oracle's Elixir
becomes useless when people chain cc her

When your team is engaging in a teamfight, your first priority is to burst down the ADC in the back while their team is fighting your team. Your hitlist should be : ADC>APC>Brusier>Tank/Jungle>Support
Remember to get out ASAP if the team is focusing you.

Keep in mind that Twilight Shroud gives sight. Imagine a scenario where you're in jungle and the whole enemy team is chasing you. Try to lay down a shroud in wraith camp/wolves/Blue buff/Red buff/Golems and ulti toward them to escape efficently. Also twilight shrould can be placed between gaps of brush for unpredictable movements.

When farming try to use Mark of the Assassin and proc the damage because you do extra damage when attacking a marked enemy and restore energy when you proc the Mark of the Assassin

Also try to use Twilight Shroud as well because it lowers enemies armor and MR.

Thank you for reading my guide and I hope it was useful to you in a way. Please comment on the guide if you have things I should add to this or whatever you want to say. Once again, thank you for reading my first guide ever made within 30 minutes. :)