Hello, my name is Halfgamer.
I'm not really into Ranked play yet but I can guarantee you that I know how to play Kog'maw.
In the right hands Kog'maw can be a force to watch out for to it's enemies and you rack up quite the killing spree with this little void puppy.

I hope you enjoy my guide, feel free to comment and take in mind that I will update this guide frequently.

Kog'maw is a very powerfull AD carry. I've been told you can play his as an AP carry, but I think that the Void Puppy's true potential comes out with Attack Damage.

Passive: Icathian Surprise
(Innate): Upon dying, Kog'Maw starts a chain reaction in his body which allows him to move faster for 4 seconds and detonate at the end of the duration, dealing 100 + (25 x level) true damage to surrounding enemies. Not a very great passive, but it can finish off enemies that are low. This doesn't mean you have to go suicide bomber all the time.

Q: Caustic Spittle
(Passive): Permanently increases Kog'Maw's attack speed. This is the only reason why I take Caustic Spittle at 4.
(Active): Kog'Maw launches a corrosive projectile at a single target which deals magic damage and reduces its armor and magic resist for 4 seconds. Not very usefull, and not very strong.

W: Bio-Arcane Barrage
(Active): For 8 seconds, Kog'Maw's autoattacks gain range and deal additional magic damage which is equal to a percentage of the target's max health. The damage caps at 100 against minions and monsters. A very usefull skill to finish off fleeing enemies and to harras.

E: Void Ooze
Active): Kog'Maw launches a projectile in a line in front of him which deals magic damage to all enemies it passes through and leaves a trail for 4 seconds which slows enemies who stand on it.
Good to harras early-game and to catch fleeing enemies.

R: Living Artillery
(Active): Kog'Maw launches a living artillery shell to a target location at a great distance which falls after a 0.6 second delay, dealing magic damage to all enemies hit and revealing them for 4 seconds. It deals 2.25 times as much base damage to champions. A very good ultimate for sniping fleeing enemies or checking brush. Due to the low cooldown and initial mana cost you can really spam this ability.

Pros
- Very Long-ranged
- Great 1 on 1 fighter
- Passive can finish of enemies is you died.
- It's a damn puppy!

Cons
- Very squishy the entire match
- Ult needs some skill to land effectively
- Doesn't have a good escape method
- Can get in mana problems

For Items I always start with the standard boots + pots. Once I got around 1300 gold I recall to upgrade my boots to Berserker's Greaves and to buy a Doran's blade. After this you should have enough damage to start killing. On my second recall I buy a B.F sword, if you can't afford B.F yet, buy a pickaxe. We want to rush Infinity as soon as possible. When midgame starts you should already have Berserker's, Infinity and a Phantom Dancer. Teamfights will start to occur more often so be prepared. I'll put a strategy for teamfights further in the guide. Now things get optional, if you feel like getting Critical strikes, get Statikk Shiv or Youmumu's. I prefer Stakkiv over Youmumu's because of the nice magic damage you get with the passive. If you want some survivability and attack damage, get Black Cleaver.

Always try to stay in the back of the teamfight, Kog'maw is very good at focusing one single enemy at a time and you should take advantage of that! Always let your tank lead the charge and start the teamfight, otherwise Kog'maw is nothing for you. You should start with the supports and then target the squishies. While your team takes distracts the tank and AP Caster you take out their AD and Support. If you get low and the enemy is low don't run away unless you already died alot. Teamfights are one of the only moments you really can use Icathian Surprise!

Tricks:
To farm easy you can shoot a living artillery shell into the wave and then finish them off with Void Ooze.
Use Living Artillery to check brush if enemies are MIA.
Alot of people understimate the true range of Kog'maws ult.

Tips:
Practice sniping fleeing enemies in bot games or with friends. It will really help!
Some enemies will recall even if they're still within Kog'maw's range. Take advantage of it!

This was my guide to Kog'maw: the Mouth of The Abyss. I hope you liked it!