This build will make kog'maw so lineable ,because of the health regen of Strenght of Spirit and doran's blade life steal.

And the ability to solo Baron and the Dragon so fast are great.

I recommend you take mid if you can.

- Upon dying, Kog'Maw starts a chain reaction in his body which causes him to detonate after 4 seconds (he can move in this time), dealing 100 + (25 x level) true damage to surrounding enemies. Kog'Maw will gain additional movement speed upon entering this state

Run at people when you die, you blow up and deal true damage (cannot be mitigated). I would like to point out to keep this in mind if you think you will probably die from a gank. You can turn around and get one of your gankers low enough to kill them with your passive. By no means TRY to die, but keep it it mind.
- (Passive) Increases attack speed (Active) Kog'Maw launches a corrosive projectile which deals magic damage and reduces the target's armor and magic resistance

When you use the active the passive does not go away, so there is no reason not to use it if you can. Since a recent patch it has been buffed to also reduce magic resist which makes it GREAT for this build, and you should use it every time before you start dpsing a target. I start with this first because it will help you last hit, and the mr / armor reduction is good for lvl 1 fights.
- (Active): For 6 seconds, Kog'Maw's attacks gain additional range and deal a percentage of the target's maximum health as magic damage (max 100 damage to monsters).

The wording here confused me when I first started playing Kog. From the wording it sounds like this effects his skills (caustic spittle, void ooze, living artillery) but in reality it only increases the range of his auto attacks. So do not use it to try to finish off people with your ult! :) This is the bread and butter of attackspeed Kog. This is what you should use every time you start dpsing in a fight, and to harass in lane.
- (Active): Kog'Maw launches a peculiar ooze which deals magic damage to all enemies it passes through and leaves a trail for 4 seconds which slows enemies who stand on it.

Nothing much to explain here. I will go over positioning of this later.
- Kog'Maw fires a living artillery shell at a great distance dealing damage and revealing targets for 4 seconds. (150% bonus damage to champions).

* Cost: 40 Mana (each subsequent Living Artillery in the next 6 seconds costs 40 additional mana, capping at 200 per cast)

* Area of Effect:

I just wanted to note the extra mana cost for Living Artillery's used in quick succession, and that it has a small area of effect (aoe). I will go over positioning of this skill later, it is very vital to success. After a recent patch this now scales with BOTH ap and ad, which with this build makes it great. We don't rely on this for the majority of our damage, but with this hybrid build it can do significant damage to fleeing enem

The reason for Manamune is to expand it with your ultimate, spam it every 6 seconds so you regain your mana and do it again. By doing this you get more mana and keep a steady pace with your mana regen.

These masteries are the best for Kog'maw becuase you don't need any mana regen (since you have it with the Manamune.) I also choose these masteries for the offense and the harass.

I will give you the best possibilities of spells to choose with Kog'maw.
Good Spells
Ghost + Flash: You will have great escapes and great chasing.
Ignite + Ghost: These two spells are also a good choice for the chasing and the offense.
Egh Spells:
Teleport: If you think your team doesn't have enough teleports choose this spell.
Cleanse: If this if your first time using this champion I recommend using this spell with Ghost.
Bad Spells:
Clarity: You don't need clarity you have good mana costs and good mana regen. (Nuff Said.)
Clairvoyance: Leave that spell for your support.
Rally + Smite: Really?
Revive: You don't really need this spell.
Exhaust: You already have a slow but you can also do this for the extra slow.
Fortify: Leave that for your support or tank.

Pros:
+ Good Chaser.
+ Good Harasser.
+ Good passive give you good area true damage.
+ You can use your Mana pool to kill enemys from almost your whole screen.
+ Great Burst Damage.
+ Great Skins Lion Dance/Butterfly.

Cons:
- No escapes.
- Sucks to be focused.

Good burst damage, good AoE ultimate, and good slows.

In team fights you always want to go in last especially if you're raping them early game they are going to want to focus the sh*t out of you. If they have another carry focus him down you have to show him who's Boss.

With this build you will dominate the game. If any enemy champions get away all you have to do is snipe them down with your ultimate and your done. Your attack speed will be insane with your passive and the items + the true damage you do with your W will devastate any opponents especially tanks.

With this build you will kill all the minions especially if your solo lane. Just use your ultimate a couple of times and your done.

When u get manamune you can kill minions on 2 hits will be easy

In conclusion you will rape in teamfights especially if you don't get focused.I hope you have fun raping the enemy team.

Happy Gaming :)

Would like to thank Otidrog for editing and reviewing my spelling erros and whatnot.