The usual Kog'Maw you meet is now skilled as a Hybrid, stealing percentages of life and so on - these guide will lead you through the playing of Kog'Maw as an AP-Monster, that just kicks *** over Distance and also is able to steal life percentagewise.

I will first explain the runes, masteries and items, so the real "how to play" you will find at the bottom.

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One of the basic problems playing an AP-Kog is, that you may run out of mana quite fast. The greater seal of Replenishment is one answer against this.
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To increase the damage you do with you abilities, greater Gyphs and Quintessences of Force are placed.
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For making sure that the damage goes through, greater marks of insight are used.
Later you will see, that the items and masteries will complete this.

The masteries basically underline what has been started with the runes: greater magic penetration, ability power and mana regeneration. Though for a long game, the blue buff is quite a good choice, so you can stay longer in the game without returning to base for mana. So duration of neutral monster buffs are increased here. Clarity also helps to stay longer in game.
To be quite mobile, you will use the teleport spell, toget back to position very fast after returning to base or to support your team in a dangerous situation.
Increased movement speed and reduced cooldowns help to follow enemies very fast and kill them with some artillery-shots of your ultimate.

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We start with a Amplifying Tome, that should be turned into a Kage's Lucky Pick when returning to base first to get more money over the game (you may sell it later or turn into Deathfire Grasp), and a Health Potion.

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Because usually Kog'Maw should play in the mid position, you usually can't jsut go to back and your teammates secure the tower. Also, after destroying the mid tower of your enemies you need to be quite mobile to help at bottom and top without the enemies really noticing that you left your position. That is why Boots of Mobility should be your choice. They will also help when you follow a damaged enemy.

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Rylai���s Crystal Scepter is necessary to slow enemies that want to run away, so you or your teammates can follow and kill him. It also helps if you need to run away and need to safe you or your teammates. Increased ability power and more healts are also essential supports in your play.

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With this, you have your basic equipment. Then you should focus on increasing your ability power. If played against tanks, Void Staff can help to break them. Rabadon's Deathcap and Zhonya's Hourglass will increase your ability power dramatically - though Zhonya's Hourglass can help you if you get in the middle of a fight - where you really don't belong to.

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Hextech Gunblade is mainly for the spell vampire, but also usefull if you later attack with your w-spell or need to slow an enemy more than with your spells (for example a running yi).

.. Viod Ooze (E)
In the beginning of the game, you should focus on the minions and not mainly on the enemy heroes. The ultimate creep-killing-spell of Kog'May is herefore thr Void Ooza (E). You should use it to get both lines of the 6 coming minions damaged. The 3 minions in the back can be easily killed by your ultimate, if you ability power is not high enough in the beginning.
It can also be used to slow enemy heroes (and it will deal a high damage as well of cause).
.. Bio-Arcane Barrage (W)
To kill tanks quite fast, Bio-Arcane Barrage is a really good support. With increasing your ability power, you will later need only a few shots to get them. The Ultimates does the rest then. The Barrage also helps in the beginning, if your enemy tries to damage you. Just stand behind your minions and spite at him with the Barrage. Also towers can be attacked earlier or from a higher rage, where enemy heroes usually don't want to go because they feel kind of safe under their tower.
.. Living Artillery (R)
The Living Artillery is the core spell of Kog'Maw. You can stay far away from your enemies and really distract them from what ever they are planning to do. With Rylai's Crystal Scepter it is also a great slow spell for coming or leaving enemies. The Artillery can also be greatly used to support your E-Spell for killing minions. At least with this spell, your enemies aren't safe under a tower.
.. Caustic Spittle (Q)
Caustic Spittle is the spell we less focus in the beginning, because we need our ranged-spells and spells to kill the creeps. Nevertheless, it is a great support if you want to kill a tower (higher attack speed) or use your barrage (higher attack speed and im teamfight less armor).
.. Icathian Surprise (Passive)
Just one thing to say: if they really kille you, just take them with you.

As I mentioned, Kog'Maw should be played in the mid-position, because this chamopion is all about positioning!
Don't get to close to fights, just deal your damage from far away and watch your enemies dying. After destroying the mid-tower of you enemies, you have a bit more room to move, so go and help your allies then.
If an enemy tries to run away: slow him and kill him. Your items and spells are great for this. It is even no problem to kill them far behind their tower.

In the beginning, just make sure you get level 6 for your ultimate and don't die. Damage the enemy minions with your E-Spell and spite a bit on them. This will make it nearly impossible for your opponent in the mid to come close to your tower. If you get close to his, use your barrage if needed to start damaging his tower. When you are level 6 and have your ultimate... have fun ;).
In The beginning, try to stay in mid as long as you have mana. Just use your E-Spell (Barrage only for tower or if your opponent starts to be to aggressive). Clarity will help you to stay there longer, After returning to base, use teleport to get back to mid, so your opponent doesn't have the time to damage your tower.

As soon as you have your shoes, you can go more often back to base (if needed, usually teleport and clarity refresh fast enough). As soon af you have finished the opponents mid-tower, head to bottom and the top tu support your team. Get the blue-buff if you run out of mana too fast. Use your ultimate with the Scepter to slow enemies and run behind the lines to kill enemies that run away or stay near their tower. Just make sure, that you don't get in any low-ranged fights!

Strengthen ur ability-pwer as much as possible and stay in movement away from the enemies. Switch position as often as possible and support your team where needed. Hunt running enemies, kill masses of minions with the E-spell and built up further. The Vampire in the lategame helps especially in Teamfights and helps to survive while taking incoming damage.
Also think about putting a ward near Baron, let your opponents nearly kill him and spite an artillery on it.

Play Kog'Maw in mid as a high-ranged artillery. Supports your team by fast switching your position. Kill minions with the E-spell and an arillery on the back-row of them. Keep out of low-ranged fights and punish tanks with your barrage, while you slow enemies with your artillery.

Have fun and tell everyone: "Don't mess with the beast!"