This build will heal your difficulty of playing Kog'Maw The Mouth Of The Abyss. I have had lots of difficulty mastering Kog'Maw and now i have mastered it so why not share it with mobafire :D.

Pros:
Can Kill any Enemy Late game below 10 hits
Great turret deffender
Great Harasser
Extremely long ranged ulti
Great Farmer
Great team fighter

Cons:
Very weak early game
Can be CC'ied
Low health early game

(Innate): 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.

His innate is extremely useful and u might get tripple kill if used right but dont die in purpose thinking you can kill them with your innate O.O

The reason i picked those runes is because you will need magic penetration to kill the unit or deal more damage with your spells, cooldown reduction will allow you to cast more spells in a short period of time and mp regen is a must for Kog'Maw as you will depend more on runes then on items on game because you will only focus on AS and AP

The items i have chosen are very effective once you have bought items up to bloodrazor. The reason is that MP regeneration is more important for Kog'Maw early game because it will allow you to cast more E to harass the opponent. Once you have Fiendish Codex you can jump straight to berserker greaves because you wont have any problem with mp regen any more so you will focus on movement as this champion is very slow. After you got berserker greaves then you will need something that can assist your W because you W will b around level 3 by now. Madreds Bloodrazor is a must item for Kog'Maw whether you build him AP or AS because with W giving additional 6% of opponent health as magic damage and with bloodrazor it will be 10% and that is rediculously amazing. Then you will start owning some opponents and to assist that you can now get Nashor's tooth for the extra AS and AP. AP will help you get a kill before you W duration finishes. After Nashor's tooth you can have Malady for AS and penetration. Or if you get too sqeesy then you can get Frozen mallet first for the health and ability to slow opponent for easy kill. After you have Frozen mallet you have 2 options. Get Hextech gunblade or Armor or MR depending on opponent. If the opponent is Fully loaded with DPS team then get Frozen heart or Thornmail. If the opponent is magician then banshee veil or force of nature because banshe veil can nuke some magic spell and Force of nature will work as an extra survibility because you will be regaining health while you are fighting.

If the opponent has more dodge like JAX then get Sword of the Divine.
If the opponent has insane AP then get Banshee veil or Force of Nature.
If the opponent is DPS then Thornmail or Frozen heart can be your best choice of survibility.

Early game try to stay behind your minions and farm as much as possible ultil you have Berserker greaves or your opponent can easily catch you and kill you because you are slow. Use your health potion whenever you have your health below 50%. Try to harrash your opponent with E straght when you see an opponent but make sure you are behind your minions unless they have low HP and is alone. Try to lane with someone that is strong melee or someone that can harass enemy too. Even after you get Berserker greaves i don't recommend you go onto enemy player yourself and try to kill them( nooooo nooo noo) but after you get bloodrazor then you can do that but not in 2vs1. you can also use your E to check our bushes.

By now you hopefully you will have Blood razor and Nashor's tooth which is pretty much everything you will need until late game. Now you can tackle your enemy yourself but not a team fight.

Note: Never start a team fight yourself because you are not tank and you are slow!

Now you can kill any enemy 1 on 1 hopefully. Cast your E whenever you see an enemy fleeing and activate W then Q then auto attack and if the opponent happens to flee cast E then chase them with ghost or exhaust them spamming R but make sure you are accurate because spamming R will cost you hell of a lot of MP.

Now comes the team battle. If you have Hextech gunblade then you can challenge any enemy 1 on 1 and you will win but if you have Deffensive item then you can act a bit like a tank by being the second guy that jumps in the fight. You can also have Any AP itmes and if you do then it is best to use R on enemy fight with W activated. If the opponent have tank then kill that tank at first as you are the best at tank killing. If you kill the opponent's tank then it is possible that the enemy will flee or if anyone is dumb enough to stay there then it is your advantage. Chase the enemy with your team then you can E to slow them if they are in range then if they are slowed with E and if they are hit by your auto attack then you can say that, that enemy is dead because your auto attacks slows the enemy. Use R if the enemy is running away with low health.

Note: never go in front of your team to start a fight because you are a kind of champion that just spams R then in close range kill it with the help of W.

However if the opponent is harassing you then all you need to do is wait for 1 to seperate then kill that opponent at the speed of light :D remember 1 on 1 with Kog'Maw on this build is your win but do not think you are immortal just because you got insane kills because you are vulnerable to CC.

Guinsho rageblade: This can be very useful for kog'maw if you are having trouble with massive AP mages like Ryze because it can give you additional AP and AD with extra passive AS.

Sorc Shoes: This item will vary depending on opponent. If you would like to deal more damage with your spells then buy this on a mid game but make sure you have berserker greaves first just that you can survive and use W effectively then mid game or late game you can sell berserker greaves then get sorc shoes after you have enough AS.

Archangel staff: if you are having trouble with MP or if the opponent is draning you to death and you dont have enough MP to cast E then get this for that it will give more AP and MP.

Mejai's Soulstealer: This is the item if you are really pro at killing enemies with Kog'Maw. extra 160 ap really cheap is rediculously awsome but only if you are against a champions that you can kill early game because this is only very effective early game because of its stacks that takes time to grow.

All the items listed above are the items you can get after you have Nashors tooth.

The main reason i did not include flash as my main summoner spell is because even with flash the opponent will find you no matter where you flash because your champ is really slow so i picked ghost instead because it will give you more speed to run away from opponent.

Why not AP Kog'Maw instead of AS Kog'Maw?
this is because AP Kog'Maw are vulnerable to silence and that could mean you die quickly if your ability is in cooldown or if you are silenced. I know AP Kog'Maw can do insane dmg but on tank killing it is much slower than AS Kog'Maw and is more vulnerable if the opponent has more tanky stuffs.