Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Simple build I made for Kog'maw. Great for ranked games.
Doran's Blade (X2) : Best starter items in the game, but sadly they cannot be build into anything. You can get 1 if you're doing very well in lane,but if not, you can grab 2 for extra health,damage, and lifesteal.
Berserker's Greaves : Nice attack speed for start, later you can sell for Mercury's Threads.
Madred's Bloodthirster : Attack speed,damage, armor(a little bit), and the PASSIVE! On hit, 8% of the target's health dealt magic damage. Rapes with his W. With the target's armor/magic resist, Kog'maw can do around 10% of the target's health with his W.
Infinity Edge : His only crit item. Rapes : nuff said
Frozen Mallet : You need this to non-squish yourself. The passive works very well for Kog'maw's great range which helps you chase and catch up.
Bloodthirster : His only lifesteal item. Farm lots and get tons of damage.
Last whisper : Awesome 40% armor pen. Everyone has armor, this item does not have to be only against tanks.
Pots : You eat pots if you go to a late late late game. I've had an 80 minute game once.
Best late late late late late late late late game items : 2X Phantom Dancers(replaces boots), Infinity Edge, Black Cleaver, Bloodthirster, Last whisper. (You can swap one for Madreds if you want, but I find them all necessary)
Pretty standard DPS runes
Not much seal choices for Kog'maw. He doesn't need dodge and etc. And secondary runes aren't that great so I get mana regain for his ult spam in some cases.
If you want, instead of 21/0/9, you can go 9/0/21 for the movement speed buff,mana regain,creep buff durations cd, etc.
Cleanse + Ghost is what I would recommend.
Cleanse : Removes debuffs(now removes Exhaust)
If you get targeted and get blinded,stun, etc, you'll be useless. This would help out a lot.
Ghost : Great for escaping and chasing, especially when you have a Frozen Mallet.
Flash : Would be great to do a last hit or run away.
Ignite : Can be used to finish someone, but I rarely use it for a high damage dealer like Kog'maw who can finish someone off easily.
Exhaust : If you're doing ranked games and you see a lot of DPS characters that would tear you apart, this would be a nice choice.
Spells you shouldn't get :
Heal : Meh, you have better choices.
Teleport : Not really much of a need.
Smite : He's not a jungler.
Clarity : This is a DPS build, not an AP build. Clarity is used for mages who run out of mana.
Clairvoyance : His ult can be used to show bushes already.
Rally : Rofl-nuff said
Fortify : Only taunters or junglers should use this spell.
Revive : It's ok after the speed thing buff, but you don't really need it.
Please post comments and rate my build. Thanks. =D